I do "Science Fridays" with my kids once or twice a month, and for one of them, we proved the speed of light with an old microwave and a chocolate bar. There i

I do "Science Fridays" with my kids once or twice a month, and for one of them, we proved the speed of light with an old microwave and a chocolate bar. There is a labeling plate on the microwave the specifies the frequency (~2.5ghz), and when you run it just long enough to begin melting spots of the chocolate bar, on older microwaves you can see several spots that are equidistant. Measure their separation (that's half the wavelength of the microwave), multiply by 2, multiply again by the frequency, and you have the speed of the wave. It's poor precision (it is melted chocolate and depends on there being some standing waves, so older microwaves seem better suited) but accurate. And the math is pretty easy for kids, a ruler and a calculator.