If you own an eBook reader, you know it can save you a lot of shelf space: a 4GB model can hold around 3,500 books which would otherwise weigh around 2 tons.
But is a 'full' eBook reader heavier than an empty one?
A billionth of a billionth of a gram heavier - about the weight of a DNA molecule.
You can thank Einstein and his famous equation for this one (E=MC2), explains computer scientist John Kubiatowicz in his New York Times article.
While an eBook reader holds the same number of electrons, to hold them in place requires more energy than if they were left unrestrained within the flash memory (about a billionth of a microjoule for each bit of data stored).
If energy equals mass times the speed of light squared (E=MC2), the mass equivalent of that extra energy calculates to about one 'attogram' - a billionth of a billionth of a gram.