The four-stroke engine is an internal combustion engine. The four strokes are: intake, compression, combustion, and exhaust. This is most easily remembered as "suck, squeeze, bang, blow". Intake means to pull in the fuel and air. Compression is when all of the fuel and air is squeezed. Combustion is when the fuel combusts or burns inside the cylinder creating a large increase in pressure. Exhaust is expulsion of the used gasses.