Who happened to pour through the funnel the liquid into the bottle, he knows that it is necessary from time to time funnel to lift, otherwise the liquid from it comes. The air in the bottle, unable to escape, retains its pressure fluid in the funnel. However, some of the liquid will drain down, so that the air in the bottle is slightly compressed by the fluid pressure. But confined in a smaller volume of air will be increased elasticity sufficient to equalize its pressure the weight of the liquid in the funnel. It is clear that by lifting the funnel, we are opening the compressed air outlet to the outside, and then the liquid starts to flow.
Therefore, it is very practical to arrange the funnel so that the tapered part can have longitudinal ridges on the outer surface, ridges that prevent the funnel to stick closely to the neck.