How to make a hydrometer–video showing a method simpler than in the text.

Following the text kids observe that an object settling into water displaces water. It floats at a point where it displaces an amount of water equal to its own weight–regardless of the material it is made from. It follows that the object will float higher or lower depending on the density of the liquid. Ask kids: How might we measure the density of a liquid? (Think Time) Design a boat-like object that enables us to easily observe and measure how high or low it floats. Such an instrument is called a HYDOMETER. The following video shows a simple way to make a hydrometer. The next step, not shown, is to calibrate it for different liquids, which can only be done by systematic testing.