This is a good question! We use "on" for most things: you sit "on" a stool, a bench, a couch, a bed, a rock, the ground, the stairs, etc. I think "chairs" and "seats" are the only exceptions to this rule. If a chair is hard and flat, you can use either "in" or "on." If it's a soft chair, like an armchair, you have to use "in." And we almost always use "in" with the word "seat," no matter what kind of seat it is.