http://crystalsoil.net/

I'm going with Silica gel. There's an agricultural version that farmers plow into fields that will swell to the size of marbles and comes in 50 lbs bags. Seems like an idea for a hilarious gag. At night before the sprinklers start, sprinkle a bag on someones lawn. In the morning, yard full of marbles.

What's with the little razorblade-like thing sticking out of the rail?