|In regards to your discussion of superheating water in a microwave oven, I've found that it occurs most often when (1) I reheat water that has been heated before and (2) I heat water that has sat in the cup overnight. Why does that seem to reduce the number of seed bubbles? - JS
Both processes allow dissolved gases to escape from the water so that they can't serve as seed bubbles for boiling. When you heat water and then let it cool, the gases that came out of solution as small bubbles on the walls of the container escape into the air and are not available when you reheat the water. When you let the water sit out overnight, those same dissolved gases have time to escape into the air and this also reduces the number and size of the gas bubbles that form when you finally heat the water. Without those dissolved gases and the bubbles they form during heating it's much harder for the steam bubbles to form when the water reaches boiling. The water can then superheat more easily.