The anwer to the question above would be that the sentence that contains a pun is the following one: I could not remember how to throw a boomerang, but it eventually came back to me.
In order to understand the pun, you need to know what the meaning of "boomerang" is. A boomerang is "a curve stick that, when throw in a particular way, comes back to the person who threw it". Here the pun has to do with the fact that memories can also come back to you, just as boomerangs do.
