I show my students the demonstration, then ask them to estimate the densities of the four materials in the jar (room temp water, very salty water, ice, cold water). Hint: The room temp water has a density of 1 g/cubic cm. See instructions below.
Make the ice cube by adding several drops of blue food coloring to water before freezing it in a styrofoam cup.
To make the salty water, I added 190 mL of pickling salt (non-iodized) to 1 L water. FYI - Salty water made with iodized salt will be cloudy - not clear.
Fill a jar half-way with tap-water, and let it sit to become room temp. To add the saltwater to the jar, use a tube and funnel. With the bottom of the tube positionede on the bottom of the jar, slowly pour the salty water in the funnel, forming the layer of very salty water below the room temp tap-water.