Mustachioed UC Berkeley computer science professor John Kubiatowicz told the NY Times that your Kindle gets heavier when you add e-books. Don't worry, though, you won't feel it with your hand, or with any scale that we've ever created.
When you load up your Kindle (or iPad, or smartphone) with data, the transistors in the flash memory use a trapped electron to distinguish between a 0 and a 1 (the poetic language of computers). According to Einstein's theory of relativity, E=mc² if something has energy, it has mass and weight. A small, undetectable, theoretical amount of mass and weight. We're talking about an atogram, or 10 to the -18th of a gram. Our bestest scales can only measure 10 to the -9th.
Kubiatowicz went on to say that the weight gained from adding data is only about one hundred-millionth as much as the estimated fluctuation from charging and discharging the device's battery—which is also almost undetectable here. For you ultralight commuters out there, I guess the lesson is that you should always travel with just one e-book and a 5-percent charged battery on your Kindle. Goodbye back-strain! [NY Times via The Register Hardware]