A friend brought us some hydrobubbles to play with. They start out about the size of couscous, and when you soak them in water they expand to the size of marbles. Our friend soaked these in blue food coloring.
This is what they look like when you drop them in a bowl of water.
Here they are in a bowl of water with some gravel.
They feel like marble-shaped jello cubes.
She told us that if you leave them in water for too long, they start to break apart into pieces, and that if you leave them sitting out in the sun they eventually shrink back to their original size.