system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How do coffee machines heat up water so fast?

Within just a few seconds of pouring cold water, it's able to pour hot coffee within just a few seconds. Much faster than a kettle. How?
gpt
It has to do with volume of water being heated at any one time.

A coffee machine runs a small amount of water continuously over a heating "element" - the part that gets hot when you run electricity through it. As a small stream of water passes over or next to the element, it touches so much of the element that the temperature drives up very quickly. A few teaspoons of cold water in becomes a few teaspoons of hot water out, over and over, because the drip rate is highly controlled.

However a kettle has a *large* amount of water with a *small* heating element in it, usually near or under the bottom of the water-holding part. Now the water next to the element gets heated, sure, just like above. But then "convection" (similar to currents in the ocean) causes that hotter water to move away and mix with all of the other still-cool water in the kettle. So instead of a few teaspoons driving temperature up quickly, a large reservoir constantly mixes still-cool water with freshly-warmed water.

So in the same way a piece of paper burns quickly but a stick of wood doesn't, the water in the kettle takes a lot longer to come up to temperature because there's so much more of it and it all needs to get heated up before it will boil.