Making weighing scales is a great way to teach kids about units of weight measurement, and the different ways in which mankind has weighed things in the past. While homemade weighing scales are never going to be as exact as a digitally calibrated scale, they can be made fairly accurate and very cheaply. They are excellent teaching tools for concepts such as weight, mass and volume, especially when using water weight as the counterbalance.
Drill three holes, equidistant apart, ¼ inch below the rim of each plastic cylinder. Make sure the cylinders are exactly the same size and model---they need to weigh the same as each other. This can be tested on a kitchen scale.
Cut six lengths of yarn, 10 inches long, and tie the ends through each hole in both the cylinders.
Tie the free ends of yarn on either end of the coat hanger, securing them with sticky tape so they don't move.
Hang the coat hanger somewhere where the cylinders can hang freely---a coat rack or hook from a door mantle work well. Make sure the cylinders are both hanging level at this point. If one appears heavier (lower) than the other, add a little sticky tape to the opposite side until they are level with one another.
Place the item you want to measure into one cylinder of the scale, and slowly add water to the other. When they are again hanging level, check the amount of water in the cylinder. Since 1 millilitre of water is equal to 1 gram, so you can tell the weight of the object via how many millilitres of water it took to make the cylinders hang level.
This is not an exact weighing scale, and should not be used for items requiring exact weight measurements. This scale should not be used for extremely heavy items.