> A quick method is to get a hunk of dry ice, put it in a metal container or box with a tight lid (taping the lid airtight helps), and sprinkling the grass on top. Allow it to sit tightly covered for about three days until all the dry ice evaporates.
Not only will it explode, scattering their stash to the four winds (or all over the floor), but most people who'd fall for that are probably teenagers and most likely end up drawing parental attention to their experimentation. So they'll blow up their stash and get themselves busted all at once.
Oh, now that's just mean.