I give this activity a B. I taped the three pieces of paper together and then flipped them over so that the taped side was down on the slide. I then taped the top piece directly to the top of the slide. I place a single piece of paper at the bottom to collect any run off (I should have placed paper towels). I placed a few drops of water in each water color well using a dropper. My son then used droppers and paint brushes to pick up the color and let it run down the slide. Fun, a little bit messy though.
As a preactivity, have your toddler roll balls and other objects down the slide. Talk about what happens to the objects. It seems that the objects want to find the floor, doesn't it! This is gravity!
Beyond: You could roll marbles or golf balls in paint and let them roll down the slide as well.