Students whirl the bottles to start a vortex. Similar to above, but with straw-like tubes up and down. When bottles are turned over, a fountain spray results. Because the volume in the lower bottle is fixed and air takes up space, the liquid has nowhere to go but up. An eye dropper in a bottle of water.
Students squeeze the bottle to make the eye dropper sink.
A second bottle has a challenge in which students try to link a diver with a hook into another hook. Two immiscible fluids with coloring. They compare the speed of the waves that develop. In the bottle with two fluids, the densities are nearly equal, which reduces the effect of gravity and slows the waves. Water fills the first bottle entirely but other bottle only partly.