underdog is right. There is a wad on "metal stuff" inside the breather, much like a coarse scouring pad. I took a junked one apart one time. As Czechman says, the old timers used to burn the oil residue out in a fire. Breathers would get stopped up and you'd get leaks at all kinds of places.
As an engine runs, the crankcase changes internal volume as pistons move in the cylinders. Hold your hand under the breather tube with the engine at idle and you can feel an in and out puffing.
