Create a groovy, 1970s-style lava lamp by filling a jar two-thirds full of water. Add some blue or red food coloring and vegetable oil. Shake some salt onto the oil, which is floating on the water's surface. Since the salt is denser than the water, it will make the oil sink to the bottom. Once the salt dissolves, the oil will float back to the top of the water.
Add blue food coloring to a clear plastic liter bottle filled three-fourths of the way with tap water. Add a small amount of vegetable oil, leaving some room at the top, and put the cap on. The oil will float on the blue water. Tilt the bottle back and forth to create waves. Little ones will love their ocean in a bottle.
Pour baby oil and tap water with food coloring -- your choice of colors or the child's -- into an empty 2-liter plastic soda bottle. Help the child drop a few craft beads into the bottle. The oil and water will remain separated and the beads will float in the middle.
Expand the lesson on density by layering additional household liquids. Measure 8 ounces each of honey, high fructose corn syrup, dish soap, water, vegetable oil, rubbing alcohol and lamp oil and put each into a separate cup. Slowly pour each of the liquids in the exact order stated above into the center of a graduated cylinder. Make sure that the liquids do not touch the sides of the cylinder while pouring. The liquids will layer on top of one another as they are poured in. The liquids that weigh more will sink below the liquids that weigh less.