A plastic is a material that can change its shape easily. Many things are made of plastics, usually because making them to the right shape is easy. There are many types of plastic. Some can be shaped only when they are freshly made; then they become hard. Others are thermoplastic and can be softened by heating them. These plastics can be used for 3D printing, because the plastic will become soft enough to form into different shapes, and then become hard again when they cool down. 