We say "pain in the knees" because you feel the pain inside your body.
With the phrase "on the bus" -- who knows?
on a bus, on a train, on a plane, on a bike, on a ship
in a car, in a helicopter, in a dinghy, in a canoe,
in a boat, on a boat
in a truck (the person driving), but on a truck (the goods being carried)
in a van, in a balloon
on a sledge
in a spaceship
Is there a rule about using in or on? Maybe this:
1. If the vehicle is big, we say usually say "on". And if you are riding something (bike, sledge, jetski) we say "on".
2. We use "in" with smaller vehicles. We use "in" for most types of boat; but with bigger ones you can say "on" as well.
3. I'm sure there are be exceptions to the rules above.
- For Teachers