say you sit on a couch, in terms of above elevation but in contact with the couch. you sit in a car, contained in the car. now here's something interesting: you would be on the seat, while in the car. and you would be at the store, in terms of geographical location. it depends on what you are trying to convey. 'at' tends to be more general. 'on' and 'in' tend to deal more with what you are describing about the object. for instance: saying i am on a horse and in a horse can mean two very different things. on would suggest that i am riding it like a cowboy rides a horse. in would suggest that it ate me! again, 'at' tends to be general. "josh is at the store" but we dont know what he is doing there, so we dont know if he is 'in' the shop to buy food, or standing 'on' the roof to throw water balloons at people. Hope that helps!