Ahead of time, make one blue ice cube for every child in your class. Also add some yello food color to lemonade
Read Little Yellow Little Blue by Leo Lionni to the class. Then pour each child a glass of lemonade. Tell them that you will add a blue ice cube to the lemonade and ask for predictions as to what will happen. Add lemonade and stir. My K4s love this activity. I also have an activity pack that each page describes one step and they draw what they did.