# Piston movements in four stroke cycle?

I was reading about a four stroke cycle. Here's what I understood:

1. In the first stroke, the piston starts at the top and moves down.
2. In the second stroke, the piston moves upwards.
3. In the third stroke, the piston moves down due to the combustion by spark plug.
4. In the final stroke, the piston moves up and the cycle continues.

I can understand why the piston moves down in third stroke due to the gasoline explosion. But, what moves the piston up and down in Step 1, 2, and 4?

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@VineetMenon: Exactly$^2$. On an old motor bike you have to use a kick starter, which on a vintage motor bike has a tendancy to kick back and smack you painfully on the calf. It can be a painful experience doing Physics. –  John Rennie Mar 13 '12 at 11:35