I drew a couple of hearts on the cement and we were filling them in and with a bucket munchkin would drop some water on the drawings. Not too much, but just enough to get them moist. As soon as the water falls on the chalk drawing you can see the texture and color change, even mix with other colors on the cement.
It was fun to see the water dry on the chalk and the colors change again.
Pretty soon with enough chalk layers and water the chalk on the floor felt like goopy paint to the touch. It was so much fun!