Did you know that the average double door (two car garage size) is around 200 pounds? Imagine if every time you had to open the garage, you had to lift that much weight by yourself. Many people would probably not be able to lift their garage door if that was the case.
The solution to this problem? Springs. Let us discuss torsion springs: A torsion spring is a spring that works by storing mechanical energy when it is twisted. Generally, 16'x7' garage doors are installed with torsion springs that must be turned 7 1/2 times, or 30 quarter turns. Twisting the spring this much puts enough energy into the spring so that when it connected to the door, the person will be able to lift it.
Other doors have different springs that are meant for that size and type of door. Because of this, the garage door should only weigh about 7-10 pounds when a person lifts it because the spring is doing the rest of the work!