Before a new airliner goes into service, every part of it is tested again and again. But there are two tests that are more important than all the others. In the first test, a modern airliner must fly at very high altitudes. Air must be pumped into the place so that the passengers can breathe. The metal structure of the plane has to be very strong for this reason. When the plane is filled with air, the air presses against the skin of the plane inside. The pressure on a small window, for example, is like a huge foot that is trying to get out. If a small part of the plane were to crack （爆裂）, the plane would explode in the sky. In order to test the structure of the plane, it is lowered into a huge tank of water .Then it is filled with air. The pressure inside the plane is greater than it ever will be when it is in the air. Finally, there is an explosion. Engineers can discover which part of the plane has cracked.
