Surprise Gift

Suppose u want to send a surpise gift to your friend.
U have a big box in which u can put that gift.
This box has a ring on which u can attach your lock.( Similarly like what u people have seen on a box where u put your locks).
The ring is big enough that many locks can be attached to it.
U hve many locks and every lock has a single key. ( Each lock has its own unique key)
But ur friend doesn't have duplicate keys for those locks. (neither the lock keys can be duplicated or a new key can be made from lock).

Now the problem is u hve to send ur gift in that box ( locked) to ur friend. Bt u cannot send the keys.

How will u do dat?

put that gift into that box and locked with one of your locks and send it. Your friend should then attach one of his own locks and return it.

when you receive again, you just remove your lock and send back. Now he can unlock his own lock get the gift.

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suresh