Lock on a box - Solution

Here is the solution.
- 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.

This answer did not satisfy Don Kirkham, who has a more elegant solution:

‘I am an instructor at the US Air Force Academy. The cadets here are some of the smartest young people you could ever hope to meet. My approach to teaching is not to just give them the course information, but to teach them how to use their very creative minds to solve problems. Logic puzzles are a great tool to achieve this, so I start each class with one (or more) puzzles. At the beginning of the semester, they sometimes have trouble with what most consider simple problems, but by the end of the semester, it is extremely difficult to find puzzles that they cannot solve! What is even more exciting is to watch them come up with solutions that are “better” then the book answer!

This is what happened with the “Lock on a box” puzzle. While I have seen the puzzle worded differently, which doesn’t allow their solution, I presented the puzzle just as it is printed on your site. The solution they came up with is as follows:

“Have your friend send you an open lock that only they have the key to.”

This solution reduces the cost of “sending” because only two trips are required. Also, the box only makes one trip vs. three trips in the original solution, reducing the cost even more.

While I have seen this puzzle in several places, I have never seen this solution.

Just a thought!’