It is corn syrup, a ziplock, tape, and food coloring. I poured about 6 oz of corn syrup into the bag (little over 1/4 bottle). I added some yellow food coloring(about 10 drops) on one side of the bag and a bout the same of red on the other side. I let out the air and closed the bag. I cut a piece of card stock a little larger than the bag. I taped over the top of the bag to make sure she couldn't get it open. I ran another strip of tape along the bottom so it was firmly attached to the card stock.
Then let the child finger paint with it. See my DD's little fingers mix the colors.
With my son I did one blue added 2 little fish (from DH's tackle box). I made one a dark dark blue and taped a yellow sheet of card stock on the back. I like adding stiff cardboard to the back. When it is dark blue with a yellow back when they push down the bright yellow shows. I tried candy fish but they didn't hold up (dissolved fast). I will not add rubber fish for my DD because my son would slide the fish around and I know my DD..... She will try to "dig" out the fish and end up making a hole in the bag. KWIM? The push to seal bags work better but I used what we had already.