Things expand when heated. It make the opening just slightly larger...enough to get the stopper out with no problem.

as glass is heated, it expands, making the opening larger. as it is cooled, it contracts, making the opening smaller. there is actually a science video about it. a science teacher is demonstrating it by putting an egg through the opening of a glass bottle. before the glass is heated, the egg is too large, but as the glass warms, the egg falls right in.