'F' is for Float

Frita floats upon the pond,
A happy little duck.
She never dives down under,
Into the bottom muck.

Her friends all throw her peanuts.
She eats them in a wink.
She has to catch them quickly,
Or else the peanuts sink.

Frita has some good advice
That slower ducks should note.
Moving to saltwater helps,
'Cause there the peanuts float!

Why do Frita's peanuts float in salt water but not in fresh water?

Objects float if they're lighter than the amount of liquid that they displace, or push aside. They sink if they're heavier. Frita'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 Frita's peanuts to float. It's the same reason that you float more easily in the ocean than in a lake.