I think we all understand how a plane flies, but for those who don't here is a quick explanation: The shape of the wing, flat on the bottom and curved on top, causes air to move faster across the top than the bottom. This, in turn, causes lower air pressure above the wing than below, creating lift as the high air pressure pushes harder than the low pressure air.

However, this theory doesn't work well when we turn the plane over. The plane would simply drop to the Earth. The way a plane flies upside down is to simply turn the nose of the plane upward. This tilts the wings, again causing lift. Don't forget that when a plane flies normally, it is tilted up about 4 degrees. So, when flying upside down, a plane usually goes for about 8 degrees of tilt. (Really, anything flat can fly as long as it is at the right angle.)

A plane can fly upside down indefinitely; the problem though is that it takes a great amount of thrust to keep it up. The great thing about the wing design (when it's upright) is that it's so efficient it requires much less thrust to keep it in the air.