Tim and Mel are long distance lovers. Tim has just purchased an engagement ring for Mel and wants to mail it to her. Unfortunately the only way to ensure the ring will be received is to place a lock on the package. Tim has locks and Mel has locks but neither have keys for each others locks. How can they ensure the ring isn't stolen?

Why this works

This riddle cleverly uses the concept of shared security to ensure the engagement ring remains safe. Initially, Tim secures the package with his lock, which Mel cannot open because she doesn't have the key. When Mel receives it, she adds her own lock, which only she can unlock. The package is then sent back to Tim, who removes his lock, thus allowing Mel to receive the package once more, now secured with her lock alone. This sequence of locking and unlocking ensures that neither can access the package at the same time without the other's involvement, keeping the ring safe throughout the journey!