Years back a classmate asked if flash drives get heavier the more you use it. I laughed at her. I stand corrected.

E-readers get heavier with each book

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.

It’s quite amazing how books have evolved.