The strength of any material is its ability to resist stretching, pressing together, and tearing what the scientists call tension, compression and shear. No one material is the strongest for all purpose. For example, cast iron is immensely strong in resisting steady compression, but is easily broken by a sharp hammer blow.
Some materials are made stronger by the addition of other substances. A little carbon added to iron makes it stronger and produces steel. The addition of silicon or magnesium to low strength aluminium forms a light weight, high strength alloy used for aircraft engines and other highly-stressed mechanisms. Nylon and similar plastics have introduced new standards of strength in non-metallic materials. They are made from basic substances such as coal, salt, petroleum, air and water, and are very tough and resistant to wear.