system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can we tolerate scolding water temps (shower/hot tub) only after easing into it?

You start your shower a little too far to the "Hot" side and your skin feels like it's melting. Yet you start at a moderate temp and can work your way up to maximum heat...
gpt
Your body has a lot of mechanisms to combat temperatures that are too hot. When you jump straight into hot water your body hasn't activated the necessary cooling systems.

For one example, when you are exposed to hot water your body rushes lots of blood to the area (blood vessels open up in the affected area). This brings lots of blood near the surface of the skin to absorb heat - it's also the reason why your skin turns red near when exposed to a lot of heat.

The blood absorbs the heat and can carry it away pretty quickly. When you ease into hot water the blood vessels have enough time to open fully at a rate that 'keeps up' with the rising temp. Essentially, your skin never gets hot enough to hurt because enough heat gets removed by the blood.

When you just jump straight into hot water blood isn't flowing to the affected areas yet. Therefore, your skin temperature rises quickly and may rise above the pain threshold. That's when you shriek out and try to swat the shower handles through a burning hot river of molten water.

TL;DR: Easing into water gives your body time to respond to the heat and prevent skin temperatures from rising into the pain point.