system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why some people have naturally curly hair and others straight? 

I know it's genetic but I want to know why everyone doesn't have the same.
gpt
Hair is made up of keratin fibers, just like your nails. As it forms, the protein twists and turns, folding around itself and doing some pretty weird things, but because more keratin is being formed behind them, they get pushed out at a regular rate, and this is why hair grows. 

Thing is though, in some people, the keratin containers more of 2 specific chemicals, building blocks called amino acids, than in other people. These amino acids are called cysteine and methionine, and they both contain sulphur (The stuff that gives rotten egg gas and farts part of it's terrible smell). The thing about sulphur, though, is that it likes sticking to other bits of sulphur, and when it gets stuck on the side of this windy protein fiber, the amino acid it's attached to, methionine, likes getting stuck to cysteine, and they start winding up together. This means the hair gets all twisty and turny like the fibers inside it. In normal hair, this doesn't happen much, but in curly hair, there's lots of cysteine and methionine, and so they get twistier and turnier.

When you straighten your hair, though, you split the methionine and cystenine apart by making them reaaaally hot, just like how spaghetti gets squishy and breaks apart when you boil it. When this happens, the hair stops sticking together and it goes straight again, but over time, the methionine and cysteine match up again, and that's why it doesn't last forever, just like how spaghetti can get harder again if you leave it out after cooking.

Thing is, people's genetics make them more likely to produce lots of methionine and cysteine in their hair, which is why it gets curly.