superheating effect

In physics, superheating (sometimes referred to as boiling retardation, boiling delay, or defervescence) is the phenomenon in which a liquid is heated to a temperature higher than its standard boiling point, without actually boiling. This can be caused by rapidly heating a homogeneous substance while leaving it undisturbed (so as to avoid the introduction of air bubbles at nucleation sites).

Because a superheated fluid is the result of artificial circumstances, it is metastable, and is disrupted as soon as the circumstances abate, leading to the liquid boiling very suddenly and violently-a very dangerous situation. Superheating is sometimes a concern with microwave ovens, some of which can quickly heat water without physical disturbance. A person agitating a container full of superheated water by attempting to remove it from a microwave could easily be scalded.

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P/S : Boiling water using a microwave can be very dangerous. To read more, click here