What is it made of, and what is it like?

An object is a thing with a shape, such as a spoon. A material is what it is made of, such as steel, wood or plastic. The same object can be made from different materials: a spoon can be steel, wood, plastic or silver, and each one feels and behaves differently.

Engineers describe a material by its properties. A property is something you can observe or measure, and it does not depend on how big the piece is. Here are the everyday ones:

• Hard or soft: does it resist being scratched or dented? A steel knife scratches a copper coin, but not the other way round.
• Stiff or flexible: does it spring back when you bend it only a little, or does it bend easily? A steel ruler is stiff, a rubber band is flexible.
• Brittle or tough: does it snap suddenly, or does it take a lot of abuse first? Glass is brittle; a rubber ball is tough.
• Light or heavy: how much mass is packed into each cubic metre? That is the density.
• Conductor or insulator: does heat or electricity pass through easily?

Density is the one you can calculate straight away: density = mass ÷ volume, measured in kilograms per cubic metre (kg/m³). Water is about 1,000 kg/m³, so anything with a higher density sinks in it. Typical values:

• Wood (oak): roughly 700 kg/m³, so it floats.
• Aluminium: about 2,700 kg/m³.
• Glass: about 2,500 kg/m³.
• Steel: about 7,850 kg/m³.
• Copper: about 8,960 kg/m³.

These are typical values. A real piece of wood, or a real alloy, can be a bit above or below the number.

Properties are things you can test. To compare hardness, scratch one material with another. To compare flexibility, bend strips of the same size. To compare density, weigh a piece and measure its volume. When you can measure a property, you can compare materials fairly instead of guessing.