Every year I demonstrate to my science classes how to change the density of water. With three different densities, it is then possible to decant the solution so that the different layers don’t mix together. I do this by adding different amount of salt to each of the three beakers. I’ve also done this with sugar and it works the same.
After the demonstration, I have my students do the same as a laboratory experiment. It’s usually a challenge to do this demo as one needs to be careful when decanting the solutions. To help with the decanting, I put a round piece of cardboard on the first layer and this helps to break the next solution from mixing.
A glass stirring rod works well. Also, a pencil or even a straw will also work when pouring the solutions. This lab generally starts a good discussion about density and how to calculate the density of different objects.