It's called "The MU Puzzle". The question is "Can you produce MU?"
You start with MI. All rules (listed below) are optional, and you can use any rule at any time, as long as you can apply the rule. You must not do anything outside the rules. The rules are "forward only". In other words, they can only be applied as written, and not in reverse.
You only use 3 letters: M, I, and U.
The rules are:
1. If you possess a string whose last letter is I, you can add a U on at the end.
2. Suppose you have Mx. Then you can add Mxx to your collection. Example: MI --> MII
3. If III occurs in one of the strings, you may make a new string with U in place of III.
4. If UU occurs inside one of your strings, you can drop it.
So, "Can you produce MU?"
Edited by monter, 10 July 2010 - 08:03 PM.