You know that some things float on water, while others sink. If a substance has a density that is less than water, then it will float, while if it has a density that is more than water, then it will sink.
This can be easily proved. Take pieces of paper, wood and iron and try to float them in a tub of water.
You will find paper and wood, both of whom have a density lesser than water, float on it while iron which has a density more than that of water will sink.
Making a Weighing Scale
Take a cloth hanger. Fix a knitting needle in its centre, so that it hangs down. Now hang this hanger from a nail fixed in a wooden board. Behind the knitting needle, on the wooden board, stick a piece of white paper and on that make scale markings. At the two ends of the hanger, hang a plastic cup each.
Now the scale is ready for use. You can, by putting different weights in the plastic cups, note down how the scale reading on the paper.