This is an activity that has been around a while, but without a lab write up, it isn't much help. I've written an introduction to microwaves and how they work along with data collection, a percent error calculation and a conclusion.
Students will measure the speed of light to a surprising decree of precision using a layer of marshmallows in a microwave oven. The rotating table is removed and each group heats their marshmallows 15-30 seconds. The marshmallows will rise and then sink, turning