A: Water has a property called surface tension: its surface behaves like a stretched skin that tries to shrink. When a vapour bubble forms inside hot water, surface tension tends to squeeze it. For a bubble to grow instead of collapsing, the water must be hot enough for the vapour inside to resist this squeezing.
At normal air pressure, water boils at 100º C. However stable bubbles need extra energy to overcome surface tension, so water can be heated a few degrees above 100º C without properly boiling. This condition is called superheating.
On a stove, the vessel’s base and rough spots heat first and bubbles form here. In an oven the microwaves heat the water more evenly through its volume. Most microwavable containers also have clean, smooth surfaces. As a result, water can sometimes sit in a superheated state without obvious bubbling.
If you then jolt the container or drop something like sugar or instant coffee into it, you suddenly disturb it enough for the superheated water to rapidly form lots of vapour.
When heating water in a microwave, avoid overlong heating and be careful when adding anything to it.
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