Most of the time you don't want your keys to get bent. A bent key can break off in a lock, leaving you no alternative but to pop the lock to get inside the door. But you may have reasons to want to bend a key. The bent key is a popular magic trick for one thing. You may want to be mischievous toward a friend or foe.
Stick one end of the key in a vise grip and hold the other end with pliers. Bend the key very slightly so that it doesn't break in half, then bend it back straight. Show the key to your audience and rub it lightly as you bend it before their eyes. It should pop out of place easily if you left the crease in place.
Put the key partly into the lock and turn it. The key will bend if you turn it hard enough. While playing this trick on someone, you want to be careful not to break the key off in the lock.
Warm your hands in gloves or near a heater before your take the key. With your warm fingers, hold the key at the ends, with one finger lodged underneath the point where the key narrows. Leave your fingers in place for a few minutes to warm the metal up, then apply pressure slowly. You will feel the first pop, then keep applying pressure to bend the key.
Use an aluminum key that is very soft but still opens locks. You must be very careful while opening a lock with this kind of key, but it can easily be bent if you want to show off or stop someone else from using the key. Have copies made in sturdier metal for your master copy.