Justice BRISTER
delivered the opinion of the Court,
in which Chief Justice JEFFERSON, Justice HECHT, Justice WAINWRIGHT, Justice GREEN, Justice JOHNSON, and Justice WILLETT joined.
The question in this case is not whether insurers should provide mold coverage in Texas, a public policy question beyond our jurisdiction as a court. The question instead is whether the language in an insurance policy provides such coverage — no more and no less.
The rules for construing insurance policies have been around for a long time, long before this dispute arose. Those rules require us to construe a policy according to what it says, not what regulators or individual insurers thought it said. Ambiguities in the plain language must be settled in favor of consumers, but they must appear in the policy itself — we cannot create ambiguities from previous policies, an agency’s interpretation, or a “mold crisis.”
The policy here provides that it does not cover “loss caused by mold.” While other parts of the policy sometimes make it difficult to decipher, we cannot hold that mold damage is covered when the policy expressly says it is not. Accordingly, we answer the Fifth Circuit’s certified question “No.”
I. “We do not cover loss caused by mold”
This case comes to us on a certified [746]*746question1 from the United States Court of Appeals for the Fifth Circuit, which asks us:
Does the ensuing loss provision contained in Section I-Exclusions, part 1(f) of the Homeowners Form B (HO-B) insurance policy as prescribed by the Texas Department of Insurance effective July 8, 1992 (Revised January 1, 1996), when read in conjunction with the remainder of the policy, provide coverage for mold contamination caused by water damage that is otherwise covered by the policy?2
The policy provision in question provides as follows:
We do not cover loss caused by:
(1) wear and tear, deterioration or loss caused by any quality in property that causes it to damage or destroy itself.
(2) rust, rot, mold or other fungi.
(3) dampness of atmosphere, extremes of temperature.
(4) contamination.
(5) rats, mice, termites, moths or other insects.
We do cover ensuing loss caused by collapse of the building or any part of the building, water damage, or breakage of glass which is part of the building if the loss would otherwise be covered under this policy.3
The rules for construing this provision are well settled. If a policy provision has only one reasonable interpretation, it is unambiguous and we must construe it as a matter of law.4 If an exclusion has more than one reasonable interpretation, we must construe it in favor of the insured as long as that construction is not unreasonable.5 A policy provision is not ambiguous merely because different parties or different courts have interpreted it differently.6
As with any other contract, the parties’ intent is governed by what they said, not by what they intended to say but did not.7 Moreover, in cases like this involving a standard form policy mandated by a state regulatory agency, we have held for more than 100 years that the actual intent of the parties is not what counts (as they did not write it), but the ordinary, everyday meaning of the words to the general public.8
[747]*747In this ease, it is hard to find any ambiguity in the ordinary meaning of “We do not cover loss caused by mold.” While the ensuing-loss clause that follows may be difficult to parse (a matter discussed below), few ordinary people would imagine that it changes the meaning of the first sentence to read “We do too cover loss caused by mold.”
The dissent finds this policy ambiguous, primarily by construing the preceding HO-B policy, on the basis that no change was intended when that form was dropped in 1990. Evidence of prior policies is extrinsic evidence, and thus inadmissible unless this policy is ambiguous.9 Ambiguity must be evident from the policy itself; it cannot be created by introducing parol evidence of intent.10 And while we have looked at a prior policy in deciding between reasonable constructions of a current one,11 we have never done so in lieu of construing the current one at all. Given the complexities found in most insurance policies, it is surely wiser to stick with our long-standing legal rule that insurance policies must be construed one policy at a time.
Nor can we agree with the dissent that this policy is ambiguous because the Texas Department of Insurance advances an interpretation that, while not convincing, is a reasonable alternative to our own. It is true that courts give some deference to an agency regulation containing a reasonable interpretation of an ambiguous statute.12 But there are several qualifiers in that statement. First, it applies to formal opinions adopted after formal proceedings, not isolated comments during a hearing or opinions in documents like the Department’s amicus brief here.13 Second, the language at issue must be ambiguous; an agency’s opinion cannot change plain language.14 Third, the agen-[748]*748c/s construction must be reasonable;15 alternative unreasonable constructions do not make a policy ambiguous.16 An agency’s opinion can help construe an existing ambiguity, but it cannot create one; that the Department agrees with the Fiess’s construction does not make this policy ambiguous.17
Moreover, neglecting what this policy says in favor of what the Department says it intended would raise a host of other problems. First, construing a statewide policy according to what a single regulator, insurer, or insured thought about it would bind many others without hearing what they might have intended. Second, even if no change was intended in 1990, that does not tell us what anyone intended before 1990, an issue we have never addressed. And finally, deriving intent from extrinsic evidence raises a fact question for jurors, not judges;18 while ambiguous exclusions must be construed in favor of the insured as long as that construction is not unreasonable,19 ambiguous intentions are not governed by the same legal rule.
We must of course consider an insurance policy in its entirety. But in doing so, we cannot overlook the obvious — that the policy provision here begins by stating unambiguously, “We do not cover loss caused by mold.”
II. “We do cover ensuing loss caused by water damage ...”
The Fiess’s argue that we must disregard how this policy provision starts (‘We do not cover loss caused by mold”) because of how it ends (“We do cover ensuing loss caused by water damage”).
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Justice BRISTER
delivered the opinion of the Court,
in which Chief Justice JEFFERSON, Justice HECHT, Justice WAINWRIGHT, Justice GREEN, Justice JOHNSON, and Justice WILLETT joined.
The question in this case is not whether insurers should provide mold coverage in Texas, a public policy question beyond our jurisdiction as a court. The question instead is whether the language in an insurance policy provides such coverage — no more and no less.
The rules for construing insurance policies have been around for a long time, long before this dispute arose. Those rules require us to construe a policy according to what it says, not what regulators or individual insurers thought it said. Ambiguities in the plain language must be settled in favor of consumers, but they must appear in the policy itself — we cannot create ambiguities from previous policies, an agency’s interpretation, or a “mold crisis.”
The policy here provides that it does not cover “loss caused by mold.” While other parts of the policy sometimes make it difficult to decipher, we cannot hold that mold damage is covered when the policy expressly says it is not. Accordingly, we answer the Fifth Circuit’s certified question “No.”
I. “We do not cover loss caused by mold”
This case comes to us on a certified [746]*746question1 from the United States Court of Appeals for the Fifth Circuit, which asks us:
Does the ensuing loss provision contained in Section I-Exclusions, part 1(f) of the Homeowners Form B (HO-B) insurance policy as prescribed by the Texas Department of Insurance effective July 8, 1992 (Revised January 1, 1996), when read in conjunction with the remainder of the policy, provide coverage for mold contamination caused by water damage that is otherwise covered by the policy?2
The policy provision in question provides as follows:
We do not cover loss caused by:
(1) wear and tear, deterioration or loss caused by any quality in property that causes it to damage or destroy itself.
(2) rust, rot, mold or other fungi.
(3) dampness of atmosphere, extremes of temperature.
(4) contamination.
(5) rats, mice, termites, moths or other insects.
We do cover ensuing loss caused by collapse of the building or any part of the building, water damage, or breakage of glass which is part of the building if the loss would otherwise be covered under this policy.3
The rules for construing this provision are well settled. If a policy provision has only one reasonable interpretation, it is unambiguous and we must construe it as a matter of law.4 If an exclusion has more than one reasonable interpretation, we must construe it in favor of the insured as long as that construction is not unreasonable.5 A policy provision is not ambiguous merely because different parties or different courts have interpreted it differently.6
As with any other contract, the parties’ intent is governed by what they said, not by what they intended to say but did not.7 Moreover, in cases like this involving a standard form policy mandated by a state regulatory agency, we have held for more than 100 years that the actual intent of the parties is not what counts (as they did not write it), but the ordinary, everyday meaning of the words to the general public.8
[747]*747In this ease, it is hard to find any ambiguity in the ordinary meaning of “We do not cover loss caused by mold.” While the ensuing-loss clause that follows may be difficult to parse (a matter discussed below), few ordinary people would imagine that it changes the meaning of the first sentence to read “We do too cover loss caused by mold.”
The dissent finds this policy ambiguous, primarily by construing the preceding HO-B policy, on the basis that no change was intended when that form was dropped in 1990. Evidence of prior policies is extrinsic evidence, and thus inadmissible unless this policy is ambiguous.9 Ambiguity must be evident from the policy itself; it cannot be created by introducing parol evidence of intent.10 And while we have looked at a prior policy in deciding between reasonable constructions of a current one,11 we have never done so in lieu of construing the current one at all. Given the complexities found in most insurance policies, it is surely wiser to stick with our long-standing legal rule that insurance policies must be construed one policy at a time.
Nor can we agree with the dissent that this policy is ambiguous because the Texas Department of Insurance advances an interpretation that, while not convincing, is a reasonable alternative to our own. It is true that courts give some deference to an agency regulation containing a reasonable interpretation of an ambiguous statute.12 But there are several qualifiers in that statement. First, it applies to formal opinions adopted after formal proceedings, not isolated comments during a hearing or opinions in documents like the Department’s amicus brief here.13 Second, the language at issue must be ambiguous; an agency’s opinion cannot change plain language.14 Third, the agen-[748]*748c/s construction must be reasonable;15 alternative unreasonable constructions do not make a policy ambiguous.16 An agency’s opinion can help construe an existing ambiguity, but it cannot create one; that the Department agrees with the Fiess’s construction does not make this policy ambiguous.17
Moreover, neglecting what this policy says in favor of what the Department says it intended would raise a host of other problems. First, construing a statewide policy according to what a single regulator, insurer, or insured thought about it would bind many others without hearing what they might have intended. Second, even if no change was intended in 1990, that does not tell us what anyone intended before 1990, an issue we have never addressed. And finally, deriving intent from extrinsic evidence raises a fact question for jurors, not judges;18 while ambiguous exclusions must be construed in favor of the insured as long as that construction is not unreasonable,19 ambiguous intentions are not governed by the same legal rule.
We must of course consider an insurance policy in its entirety. But in doing so, we cannot overlook the obvious — that the policy provision here begins by stating unambiguously, “We do not cover loss caused by mold.”
II. “We do cover ensuing loss caused by water damage ...”
The Fiess’s argue that we must disregard how this policy provision starts (‘We do not cover loss caused by mold”) because of how it ends (“We do cover ensuing loss caused by water damage”). We disagree; it has again long been the rule that we must read all parts of a policy together, giving meaning to every sentence, clause, and word to avoid rendering any portion inoperative.20
In Lambros v. Standard Fire Insurance Co.,21 homeowners alleged underground water cracked the slab of their home. Like the policy here, their policy excluded losses due to cracked foundations, but also stated that this exclusion “shall not apply to ensuing loss caused by ... water damage.” Justice Cadena writing for the Fourth Court of Civil Appeals found that the only reasonable construction of this clause was that it applied when an excluded risk was followed by an intervening occurrence that in turn caused an ensuing loss:
[749]*749To “ensue” means “to follow as a consequence or in chronological succession; to result, as an ensuing conclusion or effect.” An “ensuing loss,” then, is a loss which follows as a consequence of some preceding event or circumstance.... If we give to the language of the exception its ordinary meaning, we must conclude that an ensuing loss caused by water damage is a loss caused by water damage where the water damage itself is the result of a preceding cause. What is the preceding cause which gives to the exception the effect of taking the ensuing loss out of the reach of exception k [the foundation exception]? Again, the plain language of the exception compels the conclusion that the water damage must be a consequence, i.e., follow from or be the result of the types of damage enumerated in exception k. “Ensuing loss caused by water damage” refers to water damage which is the result, rather than the cause, of “settling, cracking, ... of foundations ...”22
This Court refused the application for writ of error, thus giving the Lambros opinion the same force and effect as one of our own.23
The part of the ensuing-loss clause at issue in Lambros is indistinguishable from the part at issue here. The Department of Insurance asserts that the Lambros policy covered fewer water risks and the homeowners’ claim did not involve mold. But the relevant ensuing-loss language has changed in no material respect; that Lam-bros involved a different house, different homeowners, and a different insurer does not make it distinguishable. If Lambros is still the law, then this clause too applies only to losses caused by an intervening cause (like water damage) that in turn follow from an exclusion listed in paragraph 1(f).
The Fiesses and the Department make several arguments for construing ensuing-loss clauses differently, but all would require reversing Lambros. That of course is not out of the question; our opinions are not like the law of the Medes and the Persians — unalterable once written.24 But we are bound to consider the principles of stare decisis before taking such a step.
Stare decisis has its greatest force in cases construing statutes, partly because our errors may be corrected by statutory amendments.25 Although Lambros did not construe a statute, it was the next thing to it — a mandatory policy form promulgated by a state agency that private parties could not alter. If our policy interpretation in Lambros was wrong, it is strange that insurance regulators did noth[750]*750ing to change the policy for a quarter century. Accordingly, we decline the invitation to overrule it.
III. “... caused by water damage ...”
Nor can we disregard how this policy provision starts (“We do not cover loss caused by mold”) because of how it ends (“We do cover ensuing loss caused by water damage”), as the latter is not as broad as the Fiesses suggest. By its own terms, paragraph 1(f) covers only ensuing losses from water damage, not water alone.
The parties disagree whether mold stemming from the small roof and window leaks at issue here would constitute “water damage.”26 While we have never construed “water damage” in the Texas homeowners ensuing-loss clause, the legendary Henry Friendly27 did (sitting with the Fifth Circuit by designation), and concluded that inadequate ventilation that led to condensation that eventually caused a floor to rot away did not fall within the Texas ensuing-loss clause because the rot was caused by water, not “water damage”:
We do not think that a single phenomenon that is clearly an excluded risk under the policy was meant to become compensable because in a philosophical sense it can also be classified as water damage; it would not be easy to find a case of rot or dampness of atmosphere not equally subject to that label and the exclusions would become practically meaningless. In our case the rot may have ensued from water but not from water damage, and the damage ensuing from the rot was not the damage from the direct intrusion of water conveyed by the phrase “water damage.”28
Surely Judge Friendly was correct. Mold does not grow without water; if every leak and drip is “water damage,” then it is hard to imagine any mold, rust, or rot excluded by this policy, and the mold exclusion would be practically meaningless.
The 15 risks excluded by paragraph 1(f) — rust, rot, mold, humidity, wear and tear, hot and cold weather, rats, termites, and so on — all damage a home incrementally; when they cause major damage, generally the home was not destroyed in a day. These 15 risks are also very common; construing the HO-B policy to cover them all would convert it from an insurance policy into a maintenance agreement.
Instead, the ensuing-loss clause provides coverage only if these relatively common and usually minor risks lead to a relatively uncommon and perhaps major loss: budding collapse, glass breakage, or water damage. In construing the last term, we are governed by the traditional canon of construction noscitur a sociis — “that a word is known by the company it keeps.”29 [751]*751Accordingly, “water damage” like its neighbors “building collapse” and “glass breakage” must refer to something more substantial than every tiny water leak or seep. Applying this traditional rule of construction, ordinary people would read paragraph 1(f) to provide coverage for the kinds of uncommon and catastrophic losses for which homeowners obtain insurance, not for the common maintenance items for which they do not.
We need not decide today the precise scope of “water damage” in the ensuing-loss clause, an issue not framed by the certified question. The issue we do decide is that a policy exclusion for “mold” cannot be disregarded by simply deeming all mold to be “water damage.”
III. “... if the loss would otherwise be covered under this policy”
All members of the Court affirm Lam-bros and refuse to construe the ensuing-loss clause outside its context. But the dissent would hold that the ensuing-loss clause cancels the mold exclusion of which it is a part, because its last phrase (“if the loss would otherwise be covered under this policy”) requires us to disregard paragraphs 1(f), 1(g), and 1(h) of the policy. Clearly, removing the 22 exclusions in those paragraphs would create mold coverage, but it would also create a different policy. To qualify as an ensuing loss, mold must “otherwise be covered under this policy.”
Here, the first sentence of 1(f) excludes mold, and the second sentence extends coverage to ensuing losses caused by water damage. If neither sentence said anything more, the two would conflict whenever water damage (covered) caused mold (excluded). But 1(f) resolves this potential conflict by limiting the second clause — the ensuing-loss clause — whenever it conflicts with anything else in the policy. By placing this proviso where it is, the only reasonable construction is that the second sentence (covering ensuing losses) must yield to the first (excluding mold), not the other way around.
This does not, as the dissent suggests, delete “otherwise” from paragraph 1(f). “Otherwise” when used as an adverb means “in a different way or manner; in different circumstances”;30 it does not mean we should disregard the immediately preceding sentence. Assume, for example, that the flight schedule of a certain airline stated:
We do not fly from Dallas to Denver.
We do fly from Dallas to all cities otherwise listed in this flight schedule.
No reasonable reader would think “otherwise” means we should disregard the preceding sentence and assume the airline really does fly from Dallas to Denver.
The dissent would rewrite the ensuing-loss clause here to provide coverage “if the loss would otherwise be covered under this policy not counting the exclusions in paragraph 1(f), 1(g), and (h).” But those exclusions are part of the policy; a policy without exclusions for rust, rot, mold, wear and tear, and termites is simply a different policy. This would be policy “construction” in the architectural rather than the legal sense.
Moreover, the upshot of the dissent’s construction would be that the more risks excluded in a policy containing an ensuing-loss clause, the broader coverage [752]*752would become. Paragraphs 1(f), 1(g), and 1(h) of the HO-B policy contain roughly 22 exclusions, and each has an ensuing-loss clause listing 3 intervening risks (building collapse, water damage, and glass breakage). According to the dissent, if any one of the 22 exclusions combines with any one of the 3 intervening risks to cause any of the 22 excluded losses, the loss is no longer excluded. This would mean there are only about 1,452 possible ways to turn exclusions into coverage.31 Thus, the more exclusions that are added, the broader coverage gets. This cannot possibly be a reasonable construction.
Finally, a “yes” answer to the certified question today would give ensuing-loss clauses in Texas a different meaning from what they have in most other American jurisdictions. These clauses are common in all-risk policies, and while rarely identical they share more similarities than differences.32 Accordingly, we should strive for uniformity in construing them.33 But the Fiess’s argument that an ensuing-loss clause can make an excluded loss reappear as a covered loss has been rejected by courts in Alabama,34Arizona,35 California,36 Florida,37 Illinois,38 Massachusetts,39 Minnesota,40New York,41 North Carolina,42 New Hampshire,43 Ohio,44 Pennsylvania,45 [753]*753Vermont,46 Washington,47 and Wisconsin.48 There would have to be something very peculiar about the Texas ensuing-loss clause for its results to be so very different from similar clauses used everywhere else.
V. Conclusion
Courts adhere to prior precedents for reasons of efficiency, fairness, and legitimacy.49 For more than a century this Court has held that in construing insurance policies “where the language is plain and unambiguous, courts must enforce the contract as made by the parties, and cannot make a new contract for them, nor change that which they have made under the guise of construction.”50 If the political branches of Texas government decide that mold should be covered in Texas insurance policies, they have tools at their disposal to do so; Texas courts must stick to what those policies say, and cannot adopt a different rule when a “crisis” arises.51
Accordingly, for the reasons and to the extent stated in this opinion, we answer the certified question “No.”
Justice MEDINA filed a dissenting opinion, in which Justice O’NEILL joined.