Slashdot linked up this cool article that tells you how to measure the speed of light using only chocolate chips, a microwave, and a ruler. It also finally gives me a good answer as to why microwaves tend to heat so unevenly:

There will be some melted hot spots and some cold solid spots in the chocolate. The distance between the hot spots is half the wavelength of the microwaves, and the frequency of the microwaves will often be printed on the back of the oven.

Of course, in my house the chocolate would never make it to the microwave...