A foam-like sponge, used to remove harmful objects from entering the engine through the carburetor. Some air filters have two different foams - one is thin and catches big objects such as grains of dirt and sand, and the other one catches the small objects such as dust. Air filters often come pre-oiled, but sometimes need to be manually oiled with air filter oil or WD-40 to further help catch the dust and particles. They need to be cleaned often using motor cleaner or a bowl of water.