A hydrometer is a tool for measuring the density of liquids. We made one by filling a water bottle with marbles and water, we made a hole in the cap and inserted a straw, securing it with hot glue. We had to adjust the water carefully, using an eye dropper, until the bottle floated beneath the surface, with the water level right at a mark on the straw. The total density of the bottle, straw, marbles, plus water and air in the bottle equaled 1 gram per cubic centimeter, the same as the water.
Then we added salt to the water in the cylinder, increasing the water's density, and the hydrometer floated higher. This why boats that travel from a fresh water river into a salty ocean float higher.
The Dead Sea is SO salty, and so dense, that you can even float in it sitting up!
Watch carefully as we add salt to the fresh water:
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