When people body-shame others.
Okay, storytime.
So, I'm hella skinny, and one day I was in math class, and I was talking with the girl who sat at my desk and the two boys who sat behind us, and one of the boys turns to me and goes "do you eat?" and he wasn't even joking he was fucking serious. Of course, I asked "what????" and he just goes "you're so skinny, do you even eat anything?" which actually made me feel awful. I eat so so much, I just can't gain any fat. I'm so skinny, and I can't control it, but when he made that comment I felt so bad, and every time I look in the mirror I see myself, this stick, and think "do people really think I starve myself?"
Don't body-shame anyone ever. Even if they're skinny. It makes people feel really bad.