Hydrometer is a device which is used to measure the specific gravity of fluids (density with respect to that of water). There are a number of ways, we can make a hydrometer at home or at school.
Using a Straw:
Cap one end of straw with modeling clay.
Using a plastic dropper:
Cut the tapering end of the plastic dropper (cut it enough so that BB can go into it) and then cap it with clay.
In either case, put the straw or dropper in a beaker or glass of water, with the clay end sticking up and mark the level to which it is submerged, using a permanent marker. We can change this level by adding or removing clay (or BB in case of dropper). Try to get this level near the top leaving an inch or so from the end. We can now use it to determine the specific gravity of other fluids. Just dip your home-made hydrometer in the liquid and mark the level to which the straw/dropper is submerged and comparing that with water marker, we can find the fluid's density.
We can also calibrate it using fluids of known density.
Hope this helps.