3-4 scouts

     1 "volunteer"

     Cover

     twig

     log

     pebble

     rock

     little bit of water

     bucket full of water

 

2 boys hold cover, one boy stands behind.  Boy in front describes machine to "volunteer" and has him experiment by tossing over tiwg -Log comes back. tosses pebble - rock comes back, and so forth. Tosses over a spoonful of water and a bucketful comes back.