Science

You may, at some time, have received a chain email about the dangers of boiling water in the microwave. But unlike pretty much every single chain email ever sent in the entire history of the internet, this one is actually true. 

We think of "boiling water" as "water that is bubbling a lot." But the bubbling action is just a secondary effect. Water hits the boiling stage at 100 degrees Celsius (212 degrees Fahrenheit).

Typically when this happens, the hottest part (in a pan on the stove, this would be the bottom) starts turning to gas. The gas bubbles up to the surface, cooling the water slightly as it does so.