Do you remember the Aesop Fable The Crow and the Pitcher? The crow has difficulty getting water out of the pitcher with her long skinny beak. In the end adding rocks to the water raised the water level and enabled her to drink.
Life with Moore Babies did an impromptu experiment to understand the concept. It's a great story/project combination for preschoolers-2nd grade. Be sure to check it out!
Have a great Easter!