Dream: Pocket full of rocks

I dream interesting stuff, and this one was fun...
I had 8 rocks, 4 black ones that looked like obsidian, and 4 cream colored kind of square ones. All a little bit bigger than a dice, but not much.

I could take the white ones and drop them on the ground up to about 3 feet apart, and they grew bigger, got to about 2 foot tall and around, didn't have to touch.....   and they stopped water flow! Once they were up to size, I could put the black ones on top, and add about another 18 inches of depth to the water it would retain. They'd hold somewhere around a month  if needed or could be just picked up and they'd shrink back down, go back in your pocket.

So what did I do with them? Early on in the dream I used them to divert the water away from a car that was stuck in a stream so we could get it out easier.  I also moved some water to fill up a pond faster. Someone else in the dream had a bunch of the black ones on his swales holding deeper for this storm to put more water into his soil while he could. He would take them back up when the swale was down to normal level.

I want a pocket full of temporary water retention rocks!! What cool things!!  

Careful not to drop them down a drain!

Are there materials that expand that much on hydration, I wonder?