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Engineers choose materials based on the properties the design requires: strength, weight, flexibility, waterproofness, cost, and appearance. No single material is best for everything — wood is light and easy to cut but rots in water; metal is strong but heavy and expensive; plastic is waterproof and cheap but not as strong as metal. Choosing the right material for each part of a design is a core engineering skill. The same object might use different materials for different parts: a hammer has a steel head (hard, heavy) and a wooden handle (light, absorbs shock, comfortable to grip).
Present a design challenge (build a container to keep a plant watered for a week while you are on vacation) and provide a variety of materials: paper, cardboard, plastic bags, aluminum foil, cotton cloth, sponges, tape, rubber bands. Before building, have students list the properties each material has (waterproof? flexible? strong?) and predict which materials would work best for each part of their design. After building and testing, compare predictions to results. This builds the habit of thinking about material properties before reaching for whatever is closest.
Before an engineer picks up a single tool, they face a critical question: what material should I use? This question might sound simple, but it is actually one of the most important decisions in the entire design process. Pick the wrong material, and even a brilliant design will fail.
Every material has properties — characteristics that describe how it behaves. Strength is the obvious one: how much force can it handle before it breaks? But strength is just the beginning. Weight matters — an airplane needs to be light, so aluminum is used instead of steel, even though steel is stronger. Flexibility matters — a tent pole needs to bend without breaking, so fiberglass is better than glass. Water resistance matters — outdoor furniture must survive rain, so plastic or treated wood is used instead of plain cardboard. Cost matters — you might want to use titanium, but if the budget only allows for aluminum, you engineer with aluminum.
The most interesting engineering decisions come when different parts of the same object need different properties. A hammer has a steel head and a wooden handle. The head needs to be extremely hard (so it does not dent when it hits nails) and heavy (so it delivers a strong blow). Steel is perfect. The handle needs to be light (so the overall hammer is not exhausting to swing), absorb shock (so your hand does not sting with every hit), and feel comfortable to grip. Wood is perfect. Making the whole hammer from steel would create a tool that is too heavy to use comfortably. Making it from wood would create a tool that cannot drive nails.
Engineers often combine materials to get properties that no single material provides. Reinforced concrete puts steel bars inside concrete: the concrete handles compression (pushing forces) and the steel handles tension (pulling forces). Plywood glues thin layers of wood with the grain running in alternating directions, making it strong in every direction instead of only along the grain. Fiberglass combines glass fibers with plastic resin to create something that is light, strong, and moldable.
The bottom line: there is no such thing as a "best" material. There is only the best material for this specific job. Developing the habit of listing what properties each part of your design needs — and then selecting materials that match those needs — is what separates thoughtful engineering from trial-and-error building.