system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does water in the shower go from mild to scolding hot when barely turning the knob?
gpt
The inside of most shower faucets looks like a nose.  There's one "nostril" for hot water, and one "nostril" for cold water. 

The way that the faucet works is that there's a disc with a single hole in it in front of these two nostrils. When the faucet is off, the nostrils are blocked.  As you turn the handle, the hole goes in front of the nostrils, passing cold first, then a mix, then hot. 

The hot water is always very hot, and the cold water is always cold. The temperature you get is a result of them mixing in proportion to how open they are based on where that hole in the disc is positioned. 

That's why you end up sometimes with very hot water with a small turn of the nob.  It doesn't take much to suddenly introduce a lot of hot water, or to push it far enough that only the hot water is open. As a result, there's a huge increase in temperature for a small motion.