When we pour a liquid into a bottle/container using a funnel, the air inside the bottle tries to escape, but since the funnel is plugged on to the opening of the bottle, it becomes difficult for it to do so. Since the mildly pressurized air inside the bottle imposes a back pressure for the fluid, the rate of filling becomes restricted. If the shape of the funnel stem is made as shown in the figure, the escape of trapped air becomes easy and enables rapid filling. The same applies to many situations, ranging from filling gasoline into a car tank or kerosene in to a bottle.