Place a plate with a single layer of marshmallows in a turntable-free microwave, and zap it until some of them just begin to melt. Measure the distance in centimeters between melt spots; it should be roughly 6 cm. Next, find a sign with the frequency of the microwave, usually 2450 MHz. With these two pieces of data, the speed of light can be calculated, via Velocity = Frequency × Wavelength. Chocolate chips or another substance which melts nicely may be substituted.