Look around the room. A window, a chair, a spoon, your shoes, your jumper. Every one is made of stuff — and not just any stuff. Nobody makes a window out of wood, or a jumper out of glass. Each thing is made of the right material for its job.
Here is the big idea, and it is a simple one: we pick a material because of what it is
good at. The things a material is good at are called its
A window has a tricky job. It must let you see through it to the garden outside, and it must keep the wind and rain out. What material is see-through and waterproof? Glass! That is why nearly every window in the world is made of glass. You can look right through it, but the rain stays outside where it belongs.
Now think about a lunchbox. It needs to be light so it's easy to carry, waterproof so your juice doesn't leak everywhere, and cheap so lots of children can have one. Plastic is all three at once — light, waterproof, and cheap to make. A glass lunchbox would be heavy and might smash in your bag. Plastic wins.
A saucepan sits right on a hot cooker, so it needs a material that is strong, that carries heat to cook your food, and — very important — that won't melt when it gets hot. Metal does all of that. Imagine a plastic saucepan on the cooker: it would go floppy and melt into a puddle before your dinner was ready!
A chair has a different job. It must be strong enough to hold you up without breaking, and easy to shape into legs and a back and a seat. Wood is perfect: it's strong, and you can cut it and join it into almost any shape. That's why so many chairs, tables and doors are made of wood.
Think about the tyres on a bike or a car. They need to grip the road so you don't slide, and to bend a little to give a soft ride, and to be waterproof in the wet. Rubber is grippy, bendy and waterproof — so tyres are rubber. So are wellies: rubber keeps the puddles out and grips the muddy ground.
And what about a cosy jumper? Its whole job is to keep you warm. Wool traps warm air close to your skin, like a sheep's woolly coat keeps the sheep warm — so we make jumpers, hats and scarves out of wool. A metal jumper would be cold, hard and no fun to wear at all!
Time to be the maker. Choose a job and watch the material that fits light up. Read the clue at the top first — it tells you the property the job needs — then see which material has it. A window lights up glass; a jumper lights up wool; wellies light up rubber. Try them all!
Matching works the other way too: pick the wrong material, and you get something useless — or downright funny.
Each one fails for the same reason: the material's properties don't match the job. Paper isn't waterproof. Glass isn't strong enough to bash things. Chocolate melts when it's warm. Get the property right, and the thing works. Get it wrong, and — splat.
Grown-ups have a funny saying: something completely useless is "about as much use as a chocolate teapot." Why? Because it's the perfect example of the wrong material. A teapot's whole job is to hold hot tea — and hot is exactly what melts chocolate! You'd pour the tea in, and the pot would slowly slump and drip away. It's silly, it's messy, and it's a brilliant way to remember the rule: always match the material to the job. (A teapot is far better made of china or metal, which don't melt.)
Look down at your shoe — it's a little team of materials, each doing its own job! The sole on the bottom is rubber, so it grips the floor and keeps out the wet. The soft top might be leather or fabric, so it bends as you walk and lets your foot breathe. The laces are string that ties tight, and the padding inside is soft and squishy to keep your foot comfy.
A bicycle is the same clever mix: a strong metal frame, rubber tyres to grip and soften the bumps, a plastic or rubber seat, and glass or plastic in the lights. Nobody makes the whole bike out of one material — because each part has a different job, and each job needs the right stuff.