In the following joke:
How do you hire a horse?
Stand it on four bricks.
The humor seems to come from the homophones hire and higher. Is that so? Because this is grammatically incorrect (to higher is not a verb, so how do you higher a horse has no grammatical meaning), and usually jokes using this kind of wordplay at least respect a syntactically valid structure.
Is this just a bad joke that does not respect the grammatical structure of the phrase, or is there another meaning? Looking at the collection of jokes at the end of the this page (which includes the horse joke) they all seem to respect grammaticality. I was not able to find a "joke explainer" site with the exact details of how a native English speaker is supposed to understand it.
(The reason I am asking is because this kind of joke has some educational value, e.g. at the end of a course when students are tired; such jokes may help introducing some extra vocabulary, or with memorizing phonetic rules. But if the joke is ungrammatical, this may actually confuse the students.)