If water is heated uniformly in a perfectly smooth container way past the normal boiling temperature, it may still not boil. Boiling requires a nucleation point, like a particle of dust or a scratch or other imperfection in the container around which the bubbles will form. You may have observed when you boil water in a pan over a burner that most of the bubbles come from a particular point on the pan. Even in a smooth container, water will generally boil when cooked on top of a stove, as it is not uniformly heated and the hot water rising from the bottom of the pan causes currents that break the surface tension of the water and create nucleation points on the surface.
Water which is heated past the boiling point of 212 degrees F (100 degrees C) but is not boiling is called superheated. If you are foolish enough to be carrying a container of superheated water and you jostle it even slightly or you place a spoon or a coffee crystal in it, the water will explode into steam on the spot and violently spatter your body and face. Such an encounter can easily result in third-degree burns and, of course, blindness if the eyes are involved.
Liquid heated in a smooth vessel in a microwave oven could well achieve superheated status. To avoid this, keep a non-metallic object, such as a wooden popsicle stick, in the container during the heating. This will provide a nucleation point so that normal boiling can proceed.
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