My son has been bugging me to play with cotton balls recently. He is a hands on learner that loves getting messy, so this is one of his favorite things to play with. We put one bin full of water and one full of cotton balls on the floor and just let him explore with them. His sister did join in and brought some droppers and cups along.
We talked about how soft the cotton balls felt both wet and dry. We also talked about how much water they held and how they felt heavier when wet. You could also add food coloring and talk about mixing colors. Really its all about little hands exploring.