First, add some food coloring in the water. This will help you see the water. Then, drop one grape into the cup of water, one grape into the cup of corn syrup, and one grape into the cup of cooking oil. Then, combine the cups of syrup, water and oil into the large clear container.
Density is one of the things that makes things float.
The three liquids float on top of one another because they have different densities. The syrup is the densest. The oil is the least dense. The water's density is in between the syrup and oil. All three liquids take up the same amount of space in the container.