text: Super-heating is correct, but you'd also like to know why it's more likely to happen with a microwave vs. hot plate. In addition to having to be above the boiling point, in order for an actual bubble to form, there needs to be a small particle in the water, or imperfection (such as a scratch) on the surface of the cup for the bubble to form around. This is called "nucleation". If you heat water on a hot plate, the water gets hot enough to boil first on the bottom surface, where there are usually lots of microscopic imperfections, and the bubbles form right away. In a microwave, though, the waves ignore the cup and heat the water directly. If the water has been filtered, the temperature can climb significantly above the boiling point away from the surface, but not have any way to boil. Since the cup itself if not being heated, it acts as a heat sink, absorbing heat from the water near the surface. That means the water at the surface can actually be cooler (below the boiling point) than deeper within, where super-heating occurs. The slightest jostle or thermal convection, though, can transport superheated water along the surface, or excite any incipient nucleation points that do exist in the bulk water, causing bubbles to form. Since the water is so hot there, the bubble forms explosively.