I flew to a friend's wedding and of course was just finishing the blanket on my way. I had my yarn scale with me so I could weigh remaining yarn as needed, and pulled it out when I started finishing colors of the blanket. It read zero. Crap.
Being an engineer and a broke grad student, I grabbed some screwdrivers and took it apart. It's pretty cool, there's a metal bar that is a cantilever (attached on one side only), so when there's weight on the plate, it bends where it's attached. This bending causes a small amount of stretching in the bar, and since the properties of the metal are known, there is a sensor attached to the side that measures the stretching and converts that to the number of grams that must be on the plate. Really cool.