what is the green top thing near the cover that has 2.0 engine in a 2000 beetle on 2000 Volkswagen Beetle

its a green little top i know where the water goes the fluid for wipers and i think the oil but idk what that green top is

Asked by for the 2000 Volkswagen Beetle
LEAVE IT ALONE; That is a port to use while checking your EVAP system. Leave it tight and don't mess with it.