What's this bag made of?
It's made of plastic.
What's this tin made of?
It's made of metal.
What's this tube made of?
It's made of metal. (the top is made of plastic.)
What are these bottle caps made of?
They're made of metal.
What are these shirts made of?
They're made of cotton. (cotton fabric)
What's the jumper made of?
It's made of wool. (wool fabric)
What's this box made of?
It's made of wood.
