This puzzle originates with Ed Pegg Jr (www.mathpuzzle.com), and was presented here Fill in the grid using the 2-digit primes below it, one digit per box, The 2-digit numbers are to be bounded by the heavy lines. Navigate with the arrow keys, HOME, and END; use the space bar to clear a square. When used, the primes disappear from the list, but are listed in parentheses ( ) if used more than once. Non-prime values are prevented from entry. For another of Ed's puzzles, see Tri-shade