Think digital information isn’t a physical element? Think again. Berkeley professor John D. Kubiatowicz�has proven that a Kindle loaded with digital content actually weighs more than an empty Kindle.
However, it’s not as dramatic as you might think. The extra weight of a Kindle filled with content isn’t actually measurable, since it’s so incredibly small — 10^-18 grams.
The Kindle uses transistors in its flash memory that distinguish between a 0 and a 1 by trapping electrons. So, when content is added, there are more trapped electrons, resulting in a slightly heavier weight of the device.
Here’s a better explanation:
The extra weight actually comes just from the device storing more energy. Kubiatowicz worked out that the additional energy 4GB of flash memory holds when full is 1.7 times 10^-5 joules, which translates into the weight difference.” — Geek.com
This information is definitely worth remembering if you ever want to impress your friends with your savvy tech knowledge.