Granted, it's not a lot. But adding a book to your Kindle does make it heavier, as computer scientist John Kubiatowicz discovered:

Although the electrons were already present, keeping them still rather than allowing them to float around takes up extra energy – about a billionth of a microjoule per bit of data.

Using Einstein's E=mc² formula, which states that energy and mass are directly related, Prof Kubiatowicz calculated that filling a 4GB Kindle to its storage limit would increase its weight by a billionth of a billionth of a gram, or 0.000000000000000001g.

This is roughly equivalent to the weight of a small virus, while the equivalent number of books – about 3,500 – would weigh approximately two tons.

Link -via Marginal Revolution | Photo: Flickr user jimmiehomeschoolmom