Weight Savers

E-readers are meant to let you carry entire libraries without any additional weight- but the devices actually get heavier every time a new text is downloaded.  The weight difference is unlikely to make much difference to holidaymakers' baggage allowances, however, because each new tome is about as heavy as a single molecule of DNA. 

It's because storing new data involves holding electrons in a fixed place in the device's memory.  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, it's been 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. :o)