system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does distilled water not work in a keurig machine?
gpt
Coffee makers rely on boiling water to pump the water by gravity. Basically, a column of boiling water containing bubbles is lighter than solid water, so the water in the column can go higher than the water level. This allows it to act like a fountain.

In order for the pumping effect to work properly, there has to be smooth, even boiling.

A problem can occur with very pure liquids and very clean surfaces. Bubbles tend to form more easily on impurities. In pure water, it is difficult for bubbles to form. This allows the water to become superheated (heated to more than the boiling point but without boiling). When a bubble suddenly does form, you can get a chain reaction where bubbles start forming on bubbles, leading to a sudden explosion of boiling which can spray water everywhere. This sudden and erratic boiling is called "bumping". It doesn't always happen, but it can happen and be a problem, so it's best avoided as far as practical. 

By using tap water, it is much less likely that you get this "bumping" and instead get smooth boiling which causes a smooth supply of water to the coffee.

It's the same reason why distilled water is not recommended for use in clothes irons. You want a steady supply of steam. Bumping boiling risks sudden spurts. The minerals in tap water help stop this happening.