This is a good question! Use "on" for things that don't have sides (or sometimes even a back): a stool, a bench, a couch, the floor, the ground, a bed. Use "in" for things that you get into, usually they will have sides and a back, and you usually use them alone: a chair, a car seat. A couch has sides, but you are not the only person who would sit there, so "on" is more appropriate.

Sit on a stool. Sit on a bench. I've heard many people say both "sit in a chair" and "sit on a chair", but I think it's safer to go with "sit on a chair".

Hi there It depends on the chair. You may sit in and on a chair. Definition by thefreedictionary.com: 1. (also tr; when intr, often foll by down, in, or on) to adopt or rest in a posture in which the body is supported on the buttocks and thighs and the torso is more or less upright to sit on a chair sit a horse If you notice it says "often followed by in or on" so it depends on the type of chair you are talking about and how large it is. Odd, I know. http://www.thefreedictionary.com/sit