A joke is a lot like a frog

A joke is a lot like a frog

They both die if you dissect them. Get it? Because if you dissect a frog, it would mean cutting into its insides which would kill it if it weren't already dead, and likewise, if you dissect a joke, which in this case means to dig deep into it, in the form of explaining it, the joke stop being funny, ruining the purpose of the joke and thus killing it metaphorically.