*Safety disclaimer- clearly these are a choking hazard, so if your kids aren't old enough to play with regular marbles, don't let them play with these either. They do fall apart if they are played with roughly. They are non-toxic. Funny story- my husband knows this for sure because he swallowed one on accident! We put just one in a glass cup on the counter top with some water so we could watch it expand. A few hours later, my husband grabbed the glass of water and drank it, thinking it was his glass from earlier. He didn't even notice that he swallowed one, and they are a bit slippery so I guess that's understandable.
I bought ours from CrystalWaterBeads.com. In addition to marble sized beads, they have many other shapes as well. To hydrate your water beads, you let them soak in distilled water overnight. My kids had fun watching them slowly change from tiny pebbles to big marbles.
We threw them in a big storage tote so they could play without losing them. They bounce really well and they slip and slide around too.
When you're done, I've read that you can wash them with some dish soap and store them hydrated in plastic bag for the next time. I tried it though, and they got icky a few days later.