Want to download thousands of e-books to your Kindle or Nook or other e-book reader? Are you worried about "the weight" of carrying all those books? That's not a problem, according to Professor John Kubiatowicz, a computer scientist at the University of California, Berkeley. He calculated weight difference and found that 4 gigabytes of text increases the weight of your e-book reader by a billionth of a billionth of a gram, about the mass of a single virus or DNA molecule."
I don't think that will make your airline luggage over weight.