why the potato exploded ?plz help me thanks !

Put a potato in the microwave, turn up the power and a few minutes later: pop! The potato explodes.

The secret is in the unique way microwaves work. Conventional ovens heat up the air inside the oven first then heat up the surface of the food and then heat up the inside. Microwave ovens cook the inside of the food right away, without heating up the surrounding air. They do this by using high frequency electromagnetic waves. As the waves oscillate back and forth in the oven, the particles in the food oscillate and rub against each other. This creates friction inside the food. This friction heats up the food's internal water, which boils and cooks the food. Foods with a lot of water, like pie filling, heat up faster than dry foods, like crust, which is why you burn your mouth on the moist filling.
When a potato is cooked in the microwave, the internal water boils and produces steam. Since potatoes have an outer skin, the steam builds up, creating pressure. Eventually, the built up pressure bursts the potato. Any item with a moist interior and a tough outer skin can explode in the microwave, if you don't create vents by piercing the skin before you microwave it.
I hope this can help your understanding. :)