These hardy structures were made not from iron or steel, but rather something unexpectedly soft: cotton.

这些坚固的结构并不是由铁或钢制成的，而是由一个极其柔软的材料制成：棉花。

Oak was formerly much used for building ships, but most ships are now built of iron or steel.

以前, 橡树经常用于造船, 但现在绝大部分船只都是由钢铁制成。

And what about this one: magnets are pieces of metal which attract certain other kinds of metal - for example, iron or steel.

磁铁属于金属，但它可以吸引其他金属，比如铁或者钢。