system
Given the following post, respond as if you're commenting on Reddit:
human
Every day I heat 1 cup of water in the microwave for 2m 10s. Some days it gently boils in the microwave. Other days the water remains still and then boils violently the moment I insert a kitchen thermometer. Why?

Footnote: I'm making coffee with an Aeropress.
gpt
You are (sometimes) superheating the water past its normal boiling point.  This occurs because the vessel you are using to heat the water is generally free of anything that can be used to nucleate a bubble of steam.  Without these nucleation points you can go past the thermodynamic equilibrium and heat the water past the boiling point.  As soon as you disturb the superheated water or put something in it that can act as a nucleation site like the thermometer it will vigorously boil until it cools down below the normal boiling point.

To avoid this potentially dangerous phenomenon when heating water in the microwave it is always recommended to put something in that can act as a nucleation site.  A toothpick or chopstick generally works pretty well, or anything else that is not metal and has a somewhat rough surface.