White poster board on the floor right in front of it. Make sure the sun is shining brightly through the window or I doubt this would work as well! The sun shines through the water and creates a rainbow that the kids can see. You could also pull out the crayons and let the kids trace and color the rainbow onto the poster board or paper that you're using.
Then I asked the kids what they could find at the end of the rainbow on St. Patrick's Day which led us right into a scavenger hunt! (My latest obsession...they're so much fun!)The hunt led everyone outside where there was a pot of gold hiding behind a tree! It was so nice out that we all stayed outside and enjoyed the chocolate coins and sunshine. Happy St. Patrick's Day!