Color changing flower: We took white roses and placed them in water with food coloring. The next day the children discovered that the flowers had taken on the color of the food coloring they were placed in: yellow, blue and red. The reason the petals of the roses changed colors was because the food colored water had traveled up the stem and into the petals.
Tabletop hovercraft: We hot glued a bottle cap to an old CD and then blew up a balloon and attached it to the bottle cap. When we slowly opened the bottle cap air flowed out of the balloon creating a cushion of moving air between the CD and the table. This reduced the friction allowing the CD to hover freely over the table.