Does my iPod get heavier as I add more music to it?

Dave - The only way that actually adding music to your iPod could increase its mass could be if you were somehow increasing the energy of it. The way modern iPods tend to store music is in what's called flash memory. This has lots of tiny cells inside it. You tend to trap the electrons, maybe a few hundred electrons in the little gap. That affects how current will flow through a wire until you can read it again. This is going to store some energy and maybe a miniscule amount. Probably 1/1000th of a joule at most inside your iPod. That's for the 1s so the 0s wouldn't store any energy - so half of that. According to E=MC2 it will increase the mass ever so slightly. That's a tiny amount but that's far less that the increasing mass. Perhaps when you charge up the battery but that's less than the mass of a greasy fingerprint you put on top of it.

Chris - So the answer is yes if you put some energy into the iPod - if whatever you do to it means it gains some energy because Einstein says E=MC2 it has to go into mass but the amount is going to be tiny.