Use a hose on a sunny day to split up the white light from the sun and make a rainbow.
Science content (2016 curriculum):
Physics: Light and Sound (1)
Earth/Space: Weather, Seasons, Climate Change (K, 1, 4, 7)
Science topic (2005 curriculum connection):
Earth and Space Science: Surroundings (grade K)
Physical Science: Light and Sound (grade 4)