Egg out of a Bottle

Perhaps the most challenging physics-in-the-kitchen demonstration for all ages is getting a hard-boiled egg with the shell removed out of a bottle that has an opening diameter smaller than the minimum diameter of the egg. Of course, one could cut up the egg with a knife inserted into the bottle and then pour out the pieces. However, we want the egg out whole and undamaged.

Long ago, physics professor Julius Sumner Miller, (who was Professor Wonderful on the early Mickey Mouse Club shows) was on the Tonight Show with host Johnny Carson and showed first how to get the egg into the bottle and then, taking no more than three seconds, had the same egg back in his hand. What is the procedure? (Hint: the same physics principles that put the egg into the bottle can get the egg out.)

Answer

The hard-boiled egg is in the bottle and the goal is to remove this egg without damage. When the total outward force acting on the egg exceeds the total opposing force resisting its exit, then the egg will accelerate out of the bottle. Assume that the bottle is held vertically upside down. Earth’s gravitational force acting on the egg that is, its weight and the force of the air inside the bottle both contribute to the total downward force acting on the egg. The upward force is the contact force of the bottle (which includes static friction) plus the force of the ambient air in the room.

To remove the egg, you need to create a pressure difference between the air inside and the air outside the bottle, with the greater pressure inside. Hold the bottle with its mouth near your mouth and its bottom somewhat higher so that the egg is positioned far forward, to the neck of the bottle, but not completely blocking the neck. Exhale a tremendous burst of breath into the bottle to suddenly increase the inside air pressure. The Bernoulli effect, the reduction in air pressure perpendicular to the air stream flow direction, caused by the rush of air flowing around the egg plus the increased inside air pressure, will aid in pushing the egg through the neck and mouth of the bottle. When the net force outward occurs, the egg accelerates outward. Sometimes the egg just pops out and must be caught in the mouth, and at other times the egg must be gently removed at the opening. The more vertical the bottle is, the more help one has from the gravitational force of the Earth.