What's the difference between a hippo and a Zippo?

Behind the Groan

Why the Zippo Is a Little Lighter

Zippo is a well-known brand of refillable metal pocket lighters used to create a small flame. The punchline first compares physical weight: a hippopotamus is extremely heavy, while a pocket-size Zippo weighs very little. Then “a little lighter” reveals a second meaning—the Zippo is literally a small lighter. Keep “really heavy” straightforward and pause briefly before the last phrase. That gives listeners time to hear “lighter” first as a weight comparison and then recognize it as the object itself.