Things We Use Every Day
Primary 1 · Science and Elementary Technology
Things We Use Every Day
Lesson 3 of 3
13 min
Let's begin
At the market, Byiringiro's mother pays 2,000 Frw for a big plastic jerrycan to carry water.
"Mama," says Byiringiro, "the paper bag is much cheaper. Why not carry the water in that?"
His mother laughs. "Try it," she says, "and you will walk home with a wet paper bag and no water at all."
Today you will learn how people choose the material for every single thing they make.
Nobody picks a material by accident. First you ask, what job must this thing do? Then you pick the material that can do that job.
A jerrycan has one job: hold water and carry it home without spilling.
So the material must hold water in, and it must be light enough to carry, and it must not break on the walk home.
Cloth soaks water up. Paper tears and gets soft. Glass would break. Plastic holds the water, is light, and does not break when you knock it.
That is why a jerrycan is plastic. Not by accident. By good thinking.
A pot sits on a hot fire, day after day.
Put a plastic basin on a fire and it melts into a black, smelly mess. Plastic hates fire.
But a baked clay pot has already been through a hot fire once. Fire does not hurt it. It holds the water and the beans, and it keeps the food warm for a long time.
That is why the pot for the fire is clay. (Some pots are metal. Metal does not melt in a cooking fire either. But metal gets very hot, so we lift it with a cloth.)
Look at a hoe. It is not made of one thing. It is made of two!
Two jobs, two materials, one good hoe.
Play this game in your head. It is fun, and it teaches you fast.
Every one of them fails, and every one fails for a reason you can now explain.
Tip
Two questions. Every time.
1. What job must this thing do? (Hold water? Sit on a fire? Bend around my body?)
2. Which material can do that job? (Plastic holds water. Baked clay laughs at fire. Cloth bends.)
Match the job to the material, and you have chosen well.
Give a reason for each one, then tap Show answer to check.
Why is a jerrycan made of plastic and not of cloth?
Answer: because plastic holds water in. Cloth would soak the water up and the water would be lost.
Why do we cook beans in a clay pot and not in a plastic basin?
Answer: because plastic melts in a fire. A baked clay pot has already been baked in fire, so the fire does not hurt it.
The handle of a hoe is wood. Why not make it out of metal too?
Answer: because wood is light and easy to hold. A metal handle would make the hoe heavy, and it would get hot in the sun.
Mutesi's family needs a roof that keeps the rain out. Should they use cloth or metal sheets?
Answer: metal sheets. Cloth soaks up water, so the rain would come straight through and drip inside.
Why is a window made of glass and not of wood?
Answer: because glass is clear. You can see through it, and it still keeps the rain outside. Wood would keep the rain out, but the room would be dark.
Note
What you learned today:
Quick check
Which material is best for a jerrycan?
