a question about Superheating

If the water has sat in the cup overnight, bubbles will not form easily.
There have been many reports of injury to people using microwave ovens to heat water to make hot drinks. Water heated in a microwave oven may be superheated and when objects (eg a spoon) or granulated materials (eg instant coffee) are put into it, the water may boil very vigorously or even appear to explode out of the container. The vigorously ejected boiling water can cause serious burns. Sometimes even the act of taking the container out of the oven and or putting it on the bench can cause the boiling.
The following conditions promote these potentially dangerous events:
-Using a container with a very smooth surface, such as an unscratched glass or glazed container.
-Heating for too long.
-Quickly adding a powder, such as instant coffee (or sometimes even an object to stir it).
Standing with one's face above the container makes injury more likely.
Water doesn't always boil when it is heated above its normal boiling temp (100 °C or 212 °F). The only thing that is certain is that above that temperature, a steam bubble that forms inside the body of the liquid will be able to withstand the crushing effects of atmospheric pressure. If no bubbles form, then boiling will simply remain a possibility, not a reality. Something has to trigger the formation of steam bubbles, a process known as "nucleation". If there is no nucleation of steam bubbles, there will be no boiling and therefore no effective limit to how hot the water can become.
Nucleation usually occurs at hot spots during stovetop cooking or at defects in the surfaces of cooking vessels. Glass containers have few or no such defects. When you cook water in a smooth glass container, using a microwave oven, it is quite possible that there will be no nucleation on the walls of the container and the water will superheat. This situation becomes even worse if the top surface of the water is "sealed" by a thin layer of oil or fat so that evaporation can't occur, either. Superheated water is extremely dangerous and people have been severely injured by such water. All it takes is some trigger to create the first bubble-a fork or spoon opening up the inner surface of the water or striking the bottom of the container-and an explosion follows.
Why is it dangerous?
If one litre of water is superheated by only 1 °C (ie if it is heated to 101 °C without boiling), it is in an unstable state, and it can suddenly produce about 3 litres of steam (This is only 1.9 ml of water, which does not sound very much). The rapid production of a substantial quantity of steam within the bulk of the water will cause it to boil vigorously and possibly to explode. The result is boiling water flying at speed out of the container violently.
Why is it possible to heat water above its boiling temp?
At the surface between air and water, or between steam and water, water boils at 100 °C if there is already a bubble of steam (or air) present. But in the absence of bubbles, water can be heated above 100 °C. There are two reasons. First, to make a stable bubble, a lot of water molecules in the same small area must form steam. This is improbable. Second, it takes extra energy to form the bubble itself: energy to push the water out of the way, and energy to make the surface between water and steam. Once a bubble forms (a process called nucleation), it is easy to increase its size. So the superheated water nearby evaporates very quickly, producing a large volume of steam.
Smooth containers do not have bubbles of air clinging to their sides. Rough walled or scratched containers may hold microscopic bubbles in their cracks. These become nuclei for boiling. Even a crack that is fully filled with water can be a boiling nucleus because it reduces the required area of the water-vapour surface.
I hope this can help your understanding. :)

Microwave oven is do not like to deal with old water (like from a old well), use new water.