system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the cold side of the pillow feel so good?
gpt
Your head has a lot of hot.

But it doesn't like to be so hot!

So your head gives the extra hot to the pillow. But the one side of pillow gets hot then too. It gets filled up with hot and then it can't take any more. So now your head is hot and the pillow is hot, and your head has nowhere to give it's extra hot and it's sad. 

Then you flip the pillow and your head is happy again, because it can share it's extra hot stuff with the cool side.