We were doing the 1/2 life experiment with small plastic dice that have one face painted. As these are withdrawn from the labs we count them back into bowls of 100, ready for next time. A new teacher gave the boys a bowl each and Gratnells trays to sprinkle into and then extract the ones with painted faces up. The residue is then put back into the bowl and the process is repeated. At the end of the lesson the boys just poured all the dice into 1 gratnells tray forming a critical mass and we now have no roof!