One can sit on a chair. You may also say that you are sitting in a chair if you wish to emphasize how a chair encloses the person who sits there; this would work particularly well with a large armchair. You also might say that a baby who is enclosed in his highchair is sitting in his chair -- but it is impossible to sit in a bench, because there is no idea of enclosure involved.

hello Zoe, We can help us if we speak by skype, what do you think about this??