I find that so much of humor is based on the “nature” of a character, and so telling a dog “fetch” is so repeatedly ripe for it. Dog’s can’t help it. You say “fetch” and bang, they’re off.

As the story goes, the scorpion said “No, frog, of course I won’t sting you if you carry me across the pond. If I did, then we would BOTH drown.” Upon stinging (of course), as they both sink, the frog asks why. “Because,” said the scorpion, “it is in my nature.”