This density experiment is an excellent representation of how denser fluids fall to the bottom of a cup, while lighter fluids float to the top of a cup. In this experiment, a type of syrup, water, and a type of oil are used to represent different densities. The syrup is the densest, while the oil is the lightest.
After the fluids separate themselves, because the differences in density keep them from mixing, it is then possible to experiment with solids. Solids that are denser than the oil and water, but not the syrup, will fall through the oil and water, but float on the surface tension of the syrup, but solids that are lighter than everything, including the oil, float atop the oil.
For example, a metal nut has a higher density than syrup, so it falls to the bottom of the cup. A grape has a higher density than water, but not syrup, so it floats atop of the syrup and sinks to the bottom of the water. A plastic cap as higher density than oil, but not water, so it sinks to the bottom of the oil and floats atop the water. And finally, a sponge, which is lighter in density than even oil, floats at the top of the cup.
Pretty nifty, eh? - Try this with the kids and watch their mouths drop!