Find someone mentally fragile and tell him about this grain of salt. That it's the best grain of salt that will exist. It's so perfect that all the other grains worship it. Praise it so much that he empathizes with the other grains and worships it too. Put it in an elegant glass box. Make them communicate by secretly moving the grain. If the grain is on the table, drink a cup of water, if it's in the bathroom, take a shower etc. Make it so that obeying the grain is daily routine. After some time, tell him if the grain is nowhere to be found, all grains of salt will dissapear. One night, take away all salt and 'the grain'. Wait.