Straw, modelling clay, surgical spirit, salt water, jar. Cut about 6cm off a straw and push a small blob of clay onto the end. Use a thick pencil to mark a line on the straw every 5mm. Try floating your hydrometer in ordinary water, salty water and surgical spirit. How does it float each time.
Salty water is heavier than ordinary water so it pushes harder against objects floating in it.
To float in salty water, objects need to displace less water than they do in ordinary water. So the hydrometer floats higher in salty water. In surgical spirit, the hydrometer floats at a lower level compared to ordinary water. This shows that surgical spirit is lighter than water. Stir with a spoon and keep adding salt until you feel a gritty layer building up on the bottom of the pan.