system
Given the following post, respond as if you're commenting on Reddit:
human
How come shower water at a given temperature feels lukewarm on my back, but extremely hot on my face?

I understand that the skin on the face is more sensitive than on other parts of the body, but why is that?
gpt
The explanation of the greater sensitivity of the face as relative to other body parts has to do with the density of sensory nerves. The face, the fingertips, the genitals, and some other parts of the body have a very high density of nerve endings. This is why you can differentiate between two pinpricks close together on your face, but not on your back. 

Basically when hot water hits you, your back is like this:

----ow----ow----ow----ow----ow----

while your face is like this:

--ow--ow--ow--ow--ow--ow--ow--ow--

The next thing you need to understand is HOW nerves communicate pain to the brain. Since the action potential that sends an electrical impulse down the axon to the brain is always the same, nerves cannot communicate the intensity of a sensation by varying the intensity of the nerve impulse. Instead, nerves communicate intensity by varying the frequency of impulses; the more intense a sensation the more frequent the pulses. 

Since there are more nerve endings in the surface area of the face relative to the surface area of the back, the face is able to convey the sensation of "YO, THIS WATER IS HOT!" more intensely than the back.