"Working from Einstein’s famous equation, which states that energy and mass are equivalent, Kubiatowicz worked out how much the weight of a Kindle might change as the books built up. He compared an empty four-gigabyte Kindle with a full one, in which half the electrons were trapped, requiring an extra 17 microjoules of energy. Popped into Einstein’s formula, this gives an answer of around one attogram, meaning the weight of a full Kindle was a billionth of a billionth of a gram more than a factory-fresh one. Which isn’t so bad, considering that 10,000 books – a fraction a Kindle can hold – might weigh five tonnes. An attogram, very roughly, is one tenth the weight of a small virus."