What wave causes water molecules to vibrate and can be used to cook and reheat food?

Microwaves. The electromagnetic waves are absorbed by water molecules even in food. When the molecules absorb the energy, they become excited and randomly collide with other water molecules. This creates frictional force from the constant collision with other water molecules hence generating frictional heat. The heat warms the food. Its practicability is its use in microwave ovens.  

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