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.
Read Phil's full explanation here.
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.