Question

how is the earth a big natural magnet

Answer 1

The Earth itself behaves as a natural magnet. It has a magnetic south pole which is actually located geographically north, in Canada. Likewise, its magnetic north pole is actually geographically south, in the Antarctic.

Like all magnets, natural ones attract or repel other magnets, as well as other materials such as iron and steel. The places on the magnet that produce opposite magnetic forces are called north and south poles. North poles always attract south poles, and vice-versa. However, north poles brought near other north poles (and south poles brought near other south poles) repel each other.

Answer 2

The earth also behaves like a magnet because it magnetic poles near the geographical north and south poles.