Why doesn’t one key open every door?

There wouldn’t be much point in locking your door if you knew that anyone cold come along with another key and unlock it! Locks and keys are made for keeping things safe from burglary or damage, and you should always lock up a house if you are going our, or a car if you are leaving it parked in a public place. There are many different shapes and sizes of keys, and each one fits into its own particular lock. The lock will be shaped inside so that it will only take its own key. If a key is forced into it which does not fit exactly, the lock will not turn, and the door will stay shut. Unfortunately, many burglars learn various tricks by which they can force a lock open, and so anything which is particularly valuable is usually protected by other devices, such as alarms, ‘combination’ locks, or one of the most effective deterrents a guard dog.