Copyright © 2007 Dorling Kindersley
Forces make things move, but they can also stretch things, squeeze them, and change their shape. A rubber ball changes shape when you use force to squeeze it, but it returns to its original shape when you stop squeezing. Materials that do this have elasticity. They are made up of particles called molecules that can stretch apart. Other materials, such as modelling clay, change shape easily when a force is applied, but they do not return to their original shape when the force is no longer applied. These materials have PLASTICITY.
A trampoline is made of stretchy rubber fastened to a metal frame by metal springs. When you land on a trampoline, you stretch the rubber and the springs. Both rubber and springs are elastic. As they return to their original shape, they pull back upwards and push you into the air.
Materials have plasticity when they are easily moulded into shape and do not return to their original shape when the moulding force is removed. When we talk about plastics, we usually mean various colourful materials that have been made out of chemicals produced from oil. In fact, the word plastic applies to any material that can be easily moulded into different shapes. Even metals can be plastic because, if heated, they soften and can be shaped.