A way to remember the cycle of events that causes a normal four-stroke internal combustion engine to function.
These are the illustrations for an OHV/OHC engine. They would be slightly different for a flathead engine.
+=====================+
intake valve--> --| +----+
spark plug--> -|- | +------------------
exhaust valve--> --| +----+
+=====================+
Suck
The intake valve opens, the air/fuel mixture is sucked into the cylinder.
+=====================+
--| +----+ piston moves down-->
-|- | +------------------
--| +----+
+=====================+
Squeeze
Both valves are closed, the piston moves upwards and squeezes the air/fuel mixture.
+=====================+
--| . .+----+ <--piston moves up
-|-. .| +----------------
--| . .+----+
+==^==================+
|
squeezed fuel
Bang
Both valves are closed, the spark plug sparks and ignites the air/fuel mixture, forcing the piston downwards.
+=====================+
--| * +----+ piston forced down-->
-|-**bang! | +------------------
--| * +----+
+=====================+
Blow
The exhaust valve opens, exhaust is blown out of the cylinder.
+=====================+
--|. .+----+ <--piston moves up
-|-. .| +------------------
.. --|. +----+
^ +==^==================+
| |
exhaust
Then, the whole thing starts over, several thousand times a minute.