Here's why:
* Transitive verbs require a direct object. The direct object is the thing that receives the action of the verb.
* In this sentence, "boil" is the verb, and "water" is the direct object. The verb "boil" is acting on the water, causing it to change state.
Intransitive verbs, on the other hand, do not need a direct object. They express a complete action without acting on anything else.
For example: "The water boils" is an intransitive sentence because the verb "boils" doesn't have a direct object.