This is an awesome science activity where the students use water and different saturations of salt water (densities) to make a rainbow in a straw.
Fill 5 test tubes with different colored water(with different amounts of salt added) and have one empty test tube for waste. Have the students use a clear straw to try different combinations of water levels. Least dense color will be on top and most dense on the bottom. The students enjoy the challenge of finding and describing density.