A few more thoughts:

Did you ever think about the fact that if you get on a horse you get on top of it, but if you get on a bus or a train you get inside? When you go to bed you actually climb inside and sleep, but if you go to town simply went to that location to do something. If we read the clock we say, "It is nine o'clock," but if we're looking at a watch we don't say, "It is nine o'watch" even though it technically is.