How Does This Physics Bet Work? (Day #198 MROH)

We've said it before: you just can't know too many ways to win a free drink. This time, the setup is simple: fold a bottlecap in half and place it inside the lip of a bottle set on its side. Tell them all they have to do to win a free drink is blow the bottlecap into the bottle. However, once they start, every puff of air only blows the bottlecap out of the bottle! The answer is in the air pressure. When they blow into the bottle, air pressure is increased on the inside, forcing the bottlecap out. However, this is not an impossible bet: by using a straw, a short, direct blast of air will blow the bottlecap into the bottle!