The purpose of this activity is to understand saltwater and freshwater, and what happens when they come together in a delta or an estuary!
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!
from a Windows to the Universe activity developed by Dave Mastie, a middle
and high school science teacher from Ann Arbor, MI.)