Water play is a great way to beat the summer heat and almost all kids love it! This particular one we did last year was to teach color mixing. It was to show that when red and blue mix together they create purple.
I set my daughter up outside with a bin full of water and two clear plastic cups one with red water and one with blue water. I colored the waters with food coloring, a few drops each. She poured in the red first!
Next went the blue, and the colors mixed to make purple. In the picture the water still looks blue but it was a lovely shade of purple in person.
After we did that and discussed the color theory and all that she started just playing with the water. Splashing around in it and then using the cups to scoop and pour the water from one to the other. Lots of summer fun!