Warming up water in the microwave is very dangerous. You may have heard of people who have been badly burned, or had a close call. When you remove the water it can seem entirely normal and safe. However, once you create a disturbance in the water by dumping in a powder or inserting a stir stick, the water explodes into a foam and sprays boiling water all over.
Why does this happen? Essentially, in some circumstances, water can be heated in a way that it cannot boil. Boiling is the process where liquid water vaporizes into a gas and the process keeps the temperature of the liquid fairly constant at 100 degrees Celsius. When water doesn't boil, it continues to absorb energy, heating it to extremely high temperatures. This effect is known as superheating water, and it is typically observed when microwaves are the heat source.
How can boiling not occur? Liquid water requires contact with a gas to boil. If you watch closely, when you pour water into a glass container or metal pan, there are usually bubbles of gas in the bottom and on the sides. The gas is trapped by imperfections on the surface or by certain materials that the container is made from. In normal boiling situations, the water heats to the point where the tiny bubbles enlarge as the liquid around them turn into a gas, finally forcing the bubble to rise to the top of the water and join the atmosphere. In superheating situations, there are no gas sources within the water, so the liquid is incapable of boiling in a normal fashion.
When water superheats, it enters a state where it has huge amounts of energy and will easily transform into a gas, given the opportunity. When you drop a substance into the water, it typically takes some bubbles of air with it. Once these bubbles are formed, a massive boil occurs at a rapid pace and with sufficient force to eject the water from the container.
Even more dangerous than simply heating water in a microwave is to heat it more than once. When you initially heat a container of water, if there are tiny bubbles in the water, they will contain steam that will return to liquid state after cooling. Therefore, if you heat a container of water once, allow it to cool, and heat it again, you are very likely to superheat the water.
Superheating is more likely to occur in some containers than others. The most susceptible containers are those made of glass or ceramic, which are harder to scratch and form imperfections than metal.
In some cases, if the water superheats enough, it will simply explode on its own within the microwave. I was able to produce such an event using a Pyrex container containing two cups of water and double-heated. As time went on, I could see the water's surface tremble with higher and higher amplitude until it suddenly exploded, launching most of the water over the inside of the microwave. Before removing the container, I allowed it to cool for several hours.
In summary, microwaving water places you and others in great danger. The superheating effect is very fascinating, but also one of the most dangerous physical reactions in the universe. If you want to try superheating water, do it without opening the microwave and always allow the water to cool for several hours before touching it. Any attempts to trigger an explosion outside of the microwave could very likely result in severe burns or even death.