What are valves?

valves are round T shaped things in the heads of engines. each cylander has at least 2 valves.one inlet and one exhaust. these valves seat in the heads and are controlled by the cam in the engine, the inlet valve opens to allow fuel into ignition chamber,while the exhaust valve will be closed. on the second half of engine cycle to that cylander the exhaust valve opens to allow the exhaust,(or burnt gas) to exit the engine. this all happens in every cylander in a proper sequence to make the engine run