"Or, what nerds do after Valentine's Day."
Here's a neat little kitchen science experiment from Gizmodo that'll give you an excuse to get rid of all the excess Russell Stover chocolate your thoughtful significant other bought you last-minute at the drugstore.
Basically, all you do is stick the chocolate in your microwave (with the turntable removed), zap it for about 20 seconds until it starts to melt, measure the distance between the melty spots and multiply it by a constant (the frequency of the microwaves x 2, or approximately 4.9x10^9) and bingo—you've just calculated the speed of light! So how does it work?
Well, a microwave oven heats foods by shooting electromagnetic waves with a frequency of 2.45 gigahertz at them, causing their water molecules to flip over and over, increasing energy, and heating the food around them.
So, given that your microwave has no turntable (or it is removed), and that it's not a modern unit with an inverter, the heating will be concentrated in spots that correlate to the wavelength of the microwaves.
Since wavelength, frequency, and the speed of light are all related to each other, (wavelength = speed of light / frequency), and microwave ovens use a fixed frequency, once we've got two of those variables (wavelength and frequency) we can calculate the third (speed of light).
If only Foucault had taken a keener interest in microwave cookery, his job would have been much easier! Neat!