Put some food colouring in each of the glasses filled with water (two red, one blue, one yellow)
Arrange the glasses in the order shown in the picture below (Red water, empty glass, blue water, empty glass, yellow water, empty glass, red water)
Fold several pieces of kitchen towel and bend over the glasses as shown.
You will see that after a while, the water is moved through the kitchen towel, in to the empty glasses, creating ‘green’, ‘purple’ ‘orange’. All of the glasses end up having roughly the same amount of water in them.
The kids really enjoyed this as it was very visual and they loved the colours. They wrote down what they thought would happen and what did happen in their note books.