A security expert has devised the “ultimate home security” locking system for the door to his house. Instead of one lock, she has installed 8 separate locks on the door. These locks are just regular “Yale-brand” locks, where you turn the key one way, it will lock the door, and when you turn it the other way, it will unlock. She has installed these locks in different ways, so that a stranger would not know which way to lock and which way to unlock for each of the locks. Her rationale was that an ordinary household lock cannot deter professional burglars, because they would have some kind of master key or gadget that could open an ordinary lock. However, by installing 8 different locks in different ways, when she leaves the house, she would turn to lock half of the locks (ie. 4) and leave the other 4 open. She keeps the information about which locks are closed and open to herself. When an expert burglar tries to open all the locks by turning the keys, the burglar will then open half of the locks (4) and close the other 4 – the door is still secured. The burglar, being an expert, after the first try, would realize that some locks are closed and some locks are open, although she does not know which and how many are closed. Of course, if the burglar knows which locks are open, she would not touch those and just turn on the ones that are closed.