Why does an egg (with the shell) burst when cooked in a microwave oven?

 Microwave radiation is generated in an electronic tube called a magnetron, and passes along what's called a wave-guide into the oven cavity.The microwaves are absorbed by foods — a characteristic that make them ideal for cooking. The microwave energy transmitted in a microwave oven is directed toward the centre of the compartment. The highest absorption factor for microwave energy is water. The water absorbs the energy and becomes agitated and this molecular level agitation is the friction that heats up food

When microwaved, different components in an egg expand at different rates, which can result in the egg exploding. White portion of egg contains a high proportion of water and yolk contains a high proportion of fat. Microwaved eggs can reach temperatures much higher than if they were simply boiled in water at 100 degrees Celsius. At these elevated temperatures, water inside the egg, mostly in the white albumen, vapourises — even as the albumen solidifies. If the pressure inside the egg exceeds the breaking strength of the shell, the egg will explode.

Using a wooden pick or tip of a knife to break the yolk membrane of an unbeaten egg before micro cooking to allow the steam to escape, can help prevent the explosion. Covering cooking containers with a lid, plastic wrap or wax paper encourages even cooking and (if we forget to prick the yolk) helps to confine the explosion Courtesy : The Hindu