This is the coolest stuff. I had never seen this.
Food Coloring (not necessary)
Start with cornstarch. (No specific amount) Add water slowly and mix with your hands. You will know when it is enough water.
This looks like a liquid substance, but it feels like chalk. Once you have it in you hand it turns to liquid again.
Fun and Messy
This was also an easy clean up. We played with it outside. When we finished I simply sprayed the mess with a hose.