Few things in life are quite as satisfying as standing in a shower that's exactly the right temperature.

But that heaven never lasts forever. The water starts to get a little cooler, so you turn the temperature knob the tiniest bit, and suddenly, you're being scalded in places you really never wanna be scalded.

So, you try to turn the knob back, and then you're being drenched in ice water. There's plenty of room for the knob to turn, though, so why is there only one minuscule spot where the water temperature is fit for humans? The first problem is that in most buildings, the hot water is held and heated in a water heater.

Adding really hot water to cold water changes the temperature more than adding the same amount of water that's just warm. So, if the water is too hot, tiny changes in how much hot water is sent your way can lead to big changes in the temperature of the mixed water hitting your body, and the water in water heaters is usually very hot. It's generally set to around 50°C to kill bacteria, although, you can often turn that down to give you a wider range of acceptable shower temperatures, assuming you're OK with the risk of said bacteria staying alive.

But even with water that isn't too hot, the spots on your temperature knob still aren't guaranteed to give you the same water temperature. For one thing, small water heaters might not hold enough hot water at any one time for a long hot shower. Newer buildings get sometimes get around this problem by heating their hot water on demand, instead of holding it in a water heater.

But a lot of plumbing doesn't actually check the temperature of the hot-cold mix. It just opens the hot and cold pipes by the right amount. So, if someone uses some of the cold at another faucet while you're in the shower, you get a burst of just hot water hitting your skin.

Combined with a water heater that's set to high, that makes the perfect shower temperature a tiny moving target that's almost impossible to hit. This moving target problem can mostly be solved by installing a pressure balancing valve, which will turn down the hot water if the cold suddenly slows down or disappears. Even newer valves, called thermostatic mixing valves, give you added control over both the temperature and pressure of your shower using a special kind of thermostat that changes size based on the temperature of the water around it.

If the cold water goes somewhere else, like to a toilet that's being flushed, the extra hot water makes the thermostat expand until it blocks more of the hot water from getting to you. And when the water around it is too cold, the thermostat contracts blocking more of the cold instead. So, other people in the house get to pee without burning you, which is nice.

And all you have to do is set the temperature you want by turning your shower's dial to the perfect temperature. Which means, you can spend less time adjusting, and more time relaxing... or bathing, or whatever. If you'd like to submit questions to be answered, go to patreon.com/scishow and don't forget to go to youtube.com/scishow and subscribe. (Outro)