Explaining a joke is like dissecting a frog.

You understand it better, but it dies in the process.

See what I did there was use the frog as an analogy to show that exposing the inner workings of a joke would essentially deprive it of its life in that it's not funny anymore. I'm drawing a parallel (and so is E. B. White) to how you basically kill a frog when dissecting it to better understand the functioning of its inner body parts, since there is now little left in the joke to laugh at.