Is there any puzzle or game which cannot be decided in non-polynomialpolynomial time and whose problem description can be easily understood by a kid? If it is a fun puzzle so much the better.
I want to demonstrate to a high school student the difference between P and NP class of problems using an example. Also, if there's some simple algorithm to show that it can be decided in polynomial time by a non-deterministic machine that would be a great bonus.