A closed container with warm water and a small pot to collect the evaporated, then re-condensed, water shows how freshwater lakes are made from salty ocean water.
Science content (2016 curriculum):
Earth/Space: Water cycle, Water conservation (2)
Science topic (2005 curriculum connection):
Earth and Space Science: Air, Water and Soil (grade 2)
Earth and Space Science: Weather (grade 4)
Physical Science: Properties of Matter (grade 2)