Kid: Mom, do trees poop?
Mom: Yes. That is how we get #2 pencils.
Listen up, buttercup. You're so dense, light bends around you. This joke is all about wordplay, see? The kid asks if trees "poop," meaning do they have waste. The mom cleverly says yes, because pencils are made from trees, and they're labeled #2, which sounds like "number two," a common euphemism for poop. Get it now, genius?