Place one layer of marshmallows in a casserole dish. The dish should be completely covered in marshmallows. Put the dish in the microwave without a turntable and cook on low heat. Continue heating the marshmallows until the layer starts to melt in 4 or 5 different places. Microwaves do not cook evenly, so you have to allow for several places to begin melting.
Take the dish out of the microwave and observe the melted spots. Use a ruler to measure the distance between the melted marshmallows. Check the back of the microwave for its frequency. There will be a consistent distance between the melted spots of marshmallow and this distance is half the wavelength of the microwave. Finally, all you have to do is multiply the microwave frequency by the wavelength. The product is the speed of light!
Did you try this? Tell us in the comments!