What is a cam shaft?

http://en.wikipedia.org/wiki/Cam_shaft

all internal combustion, and steam engines that have pistons require valves and the cam shaft, through pushrods, open and close the valves. the cam shaft is a rod of metal that has a cam or lode for each valves pushrod. the rotation of the cam is mechanically connected to the crankshaft so the camshaft, crankshaft, valves and pistons all are coordinated.

because of the cam shaft, the wheels on the bus go round and round.