NEW CHALLENGE

As of 0730, no correct, independently developed solution has been submitted. 

Here's a new cadet challenge:

There are 100 coins scattered in a dark room. 90 have heads facing up and 10 have tails up. You cannot distinguish (by feel, etc.) which coins are which. How do you sort the coins into two piles that contain the same number of tails?

This challenge will close out at 0730, Friday, 3 May.  There will be a suitable prize for the winner.  Good luck!

Coins like these