Introduction | How Does Water Get In and Out of Oceans?
Who Needs Oceans? | When Saltwater Meets Freshwater!

The purpose of this activity is to understand saltwater and freshwater, and what happens when they come together in a delta or an estuary!

Make it happen
1. Put the salt pellets into the tennis ball container. If you don’t have water-softening salt, regular table salt will do.
2. Place the golf ball on top of the salt.
3. Fill the container with tap water--stop 2 inches from the top.
4. Cap the tennis ball container and shake for several minutes (until the golf ball floats). Put a piece of paper towel over the cap while you are shaking in case the water leaks out a bit.
5. Wait a couple of minutes until saltwater clears. (If you're using regular salt, it won't clear.)
6. Slowly add tap water on top of the container, pouring it VERY SLOWLY right on top of the floating golf ball!
7. Add 2 drops food coloring and gently stir the coloring in.

What’s happening?
Where does the golf ball float?
Where does the food coloring stay?

The water on the bottom of the container is salty so it’s made of more stuff giving it a higher density than the freshwater above. The freshwater stays above the saltwater because it is less dense. Is this magic? Well, no. You can’t see density but you can see how it affects things. The golf ball stays in the middle because it has a density that is greater than freshwater and less than saltwater.

Freshwater meets saltwater wherever a river meets an ocean. This happens at estuaries and deltas. In these places the freshwater often stays above the saltwater, just like in the tennis ball container. However, waves and currents mix freshwater and saltwater. If you shake the tennis ball container, you can mix these two as well!

(Modified from a Windows to the Universe activity developed by Dave Mastie, a middle and high school science teacher from Ann Arbor, MI.)