You want to send a valuable object to a friend. You have a box which is more than large enough to contain the object. You have several locks with keys. The box has a locking ring which is more than large enough to have a lock attached. But your friend does not have the key to any lock that you have. Note that you cannot send a key in an unlocked box, since it might be copied.
How is this done?
Attach a lock to the ring. Send it to her.
She attaches her own lock and sends it back.
You remove your lock and send it back to her.
She removes her lock.
You ask the friend to send you a lock of which he has the key.
You then seal the box using that lock and send it to your friend.
As the friend still has the key he can open it.
I'm more than a fair weather friend,
On me you can depend,
To hang around at least 'til summer's end.
I serve your house, the store, the school,
I'm liked everywhere 'cause I'm so cool.
What friend is this?
I want to buy a $1.00 newspaper. I have just enough pennies $0.01, nickels $0.05, dimes $0.10, and quarters $0.25 to buy the newspaper in every possible exact-change combination of those coins.
How many coins do I have?
In order to figure out what is in a sealed box, you perform some tests.
The box seems heavy for its size and is harder to remove from a steel table than when placed on a concrete floor.
What kind of object could be inside the box?