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Most house doors are unlocked with a key. Without the right key, the lock won't open. Inside a lock is a cylinder, which runs through the doorknob and into the door. Inside the cylinder is something called a tang. When the door is locked, the tang sits partially inside and outside of the cylinder, which prevents the doorknob from being turned. When the tang is resting fully inside of the cylinder, the door is unlocked, and the doorknob can turn.
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Cylinder locks contain on average five pairs of pins. Each pin has a different height. The pairs rest in shafts that start below the cylinder, go through the middle of the cylinder and above it. The upper shafts have tiny springs resting in the top of them.
Normally, the lower pins rest in the bottom part of the shaft, and the upper pins sit halfway in the upper part and halfway in cylinder. This means the cylinder can't rotate. When a key is inserted into the cylinder, the notches in the key line up with the pins and push them up or down fully into the shafts. This frees up the cylinder to rotate. Once the cylinder rotates, it locks or unlocks the door.
Once the cylinder can rotate, it turns something called a cam. The movement of the cam frees a spring, which pushes something called a tang forward, fastening the door to the wall. This locks the door. Inserting the key and turning the cylinder the other way will withdraw the tang. Many house doors have buttons on the inside doorknob that can be pressed or turned to release the spring and tang, locking the door without a key.