Fern the Duck catches peanuts
She eats them in a wink.
She has to catch them quickly,
Or else the peanuts sink.
Fern can give some good advice
That slower ducks should note.
Moving to saltwater helps,
'Cause there the peanuts float!

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

Kids, here's how you can test Fern's theory for yourself. Use masking tape and a pencil to label two clear plastic cups as "fresh water" and "salt water". Add water to each cup until 3/4 full. Add 6 teaspoons of table salt to the salt water cup and mix with a spoon for a few minutes. Add a peanut (the kind that is already shelled) to each cup, and observe what happens. You should see Fern's theory in action: the peanut in the fresh water will instantly sink while the peanut in the salt water will float. In fact it will float all night! Objects float if they're lighter than the amount of liquid that they displace, or push aside. They sink if they're heavier. Fern's peanuts are heavier than the fresh water that they displace, so they sink. Dissolving salt in fresh water makes the same amount of liquid heavier, allowing the peanut to float. It's the same reason that you float more easily in the ocean than in a lake!

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
December 1997

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Taken From: Apples, Bubbles, and Crystals: Your Science ABCs, by A. Bennett & J. Kessler, 1996, McGraw Hill, NY.