How to Make a Hydrometer?

You can make a simple hydrometer by capping one end of a straw with about a centimeter ball made of clay. Put 2 small head nails into the bottom of the straw. Place the straw in a cup of water and note the water line, then mark with a permanant marker. Finally, use a ruler and mark the straw with a different color marker at 1 cm increments on either side of the water level mark.