Coincidentally, since you mentioned it in your first post, I received on Saturday a newsletter from my former employer which included an article on the potential danger of microwaving liquid on it's own. I am sure you take care but here is a sorry tale of somebody who didn't and the consequences which I hope nobody else will experience. This is the gist of the article.
A 26 year old man decided to have a cup of coffee. He took a cup of water and put it in the microwave to heat it up, (something that he had done numerous times before). I am not sure how long he set the timer for but he wanted to bring the water to a boil. When the timer shut the oven off, he carefully removed the cup from the oven. As he looked into the cup, he noted that the water was not boiling but suddenly the water in the cup 'blew up' in his face. The cup remained intact until he threw it out of his hand but the water had flown in his face due to the build-up of energy. His whole face is blistered and he has 1st and 2nd degree burns. He also may have lost partial sight in his left eye. Whilst at hospital, the doctor who was attending to him stated that this is a fairly common occurrence and water (alone) should never be heated in this manner, something should be placed in the cup to diffuse the energy such as a wooden stir stick, tea bag, etc (nothing metal).
MICROWAVE MANUFACTURER'S RESPONSE
Microwaved water and other liquids do not always bubble initially when they reach boiling point. They can get superheated and the superheated liquid will bubble up out of the cup when it is moved or when something like a spoon or tea bag is put into it. To prevent this from happening and causing injury, do not heat any liquid for more than two minutes per cup. After heating, let the cup stand in the microwave for thirty seconds before moving it or adding anything into it.
There followed a technical explanation which I will skip.
Must admit I have heated liquid in a microwave a number of times but in future will take great care when setting the timer!