The distinctive qualities of a material that can be tested and used to help people select the most suitable one for a particular use. Mechanical properties are determined when a force is applied to a material, for example, to test its strength, hardness, wear resistance, machinability/workability, stretch and elasticity. Thermal properties are determined when varying temperatures (for example, cold or heat) are applied to test whether it expands, melts, conducts or absorbs heat (warms up), find its boiling point, and whether its colour changes. Chemical properties relate to the chemicals a material is made up of (its composition) and how it may change because of its surrounding environment, for example, how it ages or taints; develops an odour; deteriorates; resists stains, corrosion or cracks due to heat; or is flammable. Electrical properties relate to the way a material responds if a current is passed through it or if it is placed in an electrical field, for example, whether the material conducts or resists electricity or acts as an insulator. Optical properties relate to how light reacts with a material, for example, opaqueness, transparency and reflectiveness.