Stopping color in its tracks

This experiment was hard to pull off, but very cool once we got it to work. Since salt water is denser than fresh water, you should be able to get fresh water to sit on top of salt water, rather than all mixing together. The idea is that you put clear salt water in one container and fresh water with food coloring in another container. When you poor the colored water into the container with the salt water, the colored water should sit on top of the salt water, giving the appearance that you've stopped the color in its tracks. The only problem is that the force of pouring the colored water makes it go to the bottom of the second container before rising to the top, coloring the salt water in the process. Remembering how we got a paper clip to float using surface tension, we floated a piece of cardboard on top of the salt water and carefully poured the colored water onto the cardboard. The trick worked - although we had one attempt where some of the colored water missed the cardboard, greatly diminishing the wow factor. But when it worked, it was surreal.