What do you call a sad cup of coffee? A ...

It makes you stop when you want to go, it tells you when it is your turn, you know.

Throw me, and if your aim is true, I curve through the air and return to you.

I cover your head, but I am not a hat. I am often attached to a coat. What am I?

I am your largest organ, and first line of defence, I have three different layers but the top one you know best, It keeps out water and cold out, and melanin defines its colour, It's not just humans who have me; there's a lot of things I can cover