In a nutshell, the microwaves cause water molecules inside the food or drink to vibrate at a rapid pace.
The vibrating molecules rub against each other, and the friction rapidly creates heat.
The absorption of energy in the water, fats and sugar contained in the food is called dielectric heating.
Microwave ovens are used in industrial processes for drying and curing products.
The kitchen appliances became popular in the United States in the late 1970s, after the development of inexpensive cavity magnetrons.