A Kindle loaded with books is heavier than an empty one. E-readers store data by trapping electrons and require more energy to hold those electrons in place. Using Einstein’s famous mass/ energy equation, a computer scientist found that a full, 4GB Kindle weighs one attogram, or roughly 1/10th the weight of a small virus. Basically, a filled Kindle weighs a billionth of a billionth of a gram more than a factory-fresh one. Source Source 2