My friend Phil Plait answers the question, "Which weighs more, five pounds of helium or five pounds of cheese?" But first, he has to figure out how to weigh the helium...

*This question is an old variation on the riddle, “What weighs more, a pound of lead or a pound of feathers?” The answer is neither: They both weigh a pound. But I remember hearing this riddle when I was but a wee lad, and being momentarily baffled. We humans sometimes confuse weight and density; lead is very dense, but a pound’s a pound the world around. A pound of feathers would take up a lot more space, but it would still weigh a pound.*

*But in the case of Jenny’s question, we get even more confused. After all, helium floats! If you had a balloon full of helium, and tried to weigh it on a scale, it would float away. If you could somehow tie it to the scale -- and you had a scale with the decidedly odd characteristic that it could measure numbers less than zero -- it would say the balloon has negative weight!*

*But that’s not really the case. Here’s the answer: A balloon filled with five pounds of helium would weigh exactly the same as a five pound block of cheese.*

*How can this be? Ah, let me explain.*

Read Phil's full explanation here.