What does a cigarette and a hamster have in common?
Both are completely harmless until you put it in your mouth and light it on fire.
Alright listen up buttercup. This joke is all about expectations. You think a hamster is cute and harmless, right? And a cigarette on its own ain't gonna hurt ya. But BOOM! Stick that hamster in your mouth and light it up? Instant regret, my friend. The joke plays on the unexpected danger of combining two seemingly innocent things. It is funny because you are dumb.