Age 16 to 18
This is the sort of problem that needs a lot of thinking
Present it and leave it for a week or so before returning to it
If no one has made much progress - what have they managed to
At some stage it may be worth mentioning that the solution requires
more than 20 moves.
What recording methods have people come up with?
Then the big question:
How can you know you have managed to swap the knights in the
minimum number of moves? This will certainly need a convincing