Microwaves can transform a frozen pizza into hot, melted goodness in four minutes flat, but they can't rescue your melted ice-cream sundae. To cook food, a microwave oven converts voltage into high-frequency electromagnetic microwaves. The molecules in food—especially water and fat—absorb this energy and wiggle at high speeds, causing them to heat rapidly and warm the surrounding food. Although quickly turning leftovers cold would be handy, this is a one-way operation, explains David Pozar, a professor and microwave expert at the University of Massachusetts. Microwaves can only speed up atoms, not slow them down.