A spring scale can easily be made by attaching a spring or piece elastic to a stand. A small pointer attached to the end of the spring or elastic can be used to tell you what the weight of the particular object is. Calibrating the scale becomes simple when using the metric system, as the metric system is based off of the weight and volume of water. By suspending different amounts of water on the spring, you can see how far the spring extends for different weights.

1 Liter of water = 1 kilogram = 1000 grams
1/2 Liter of water = 1/2 kilogram = 500 grams
250 Milliliters water = 1/4 Liter of water = 1/4 kilogram = 250 grams
100 Milliliters water = 0.10 Liter of water = 0.10 kilogram = 100 grams
10 Milliliters water = 0.01 Liter of water = 0.01 kilogram = 10 grams

Different springs require different amounts of force to extend, so you will need to make different scales for different amounts of weight. Thin springs and rubber bands can weight light objects, while heavier springs and elastics can weigh the heavier objects.