Explanation:
When a thin long piece of magnet is suspended freely, it comes to rest in the North-South direction. This is due to the following reasons:

Earth's Magnetic Field:
The Earth has a magnetic field, which is similar to that of a bar magnet. It has a north pole and a south pole, just like a magnet. The Earth's magnetic field is inclined at an angle of about 11.5 degrees to the axis of rotation, which means that the magnetic north pole is not exactly coincident with the geographic north pole.

Magnetic Dip:
The angle between the magnetic field and the horizontal plane is known as the magnetic dip. At the magnetic equator, the dip is zero, but as we move towards the magnetic poles, the dip increases. At the magnetic poles, the dip is 90 degrees. In the mid-latitudes, the dip is about 60 degrees.

Magnetic Moment:
A magnet has a property called magnetic moment, which is a measure of its strength. It is defined as the product of the pole strength and the distance between the poles. The magnetic moment of a magnet is directed from the south pole to the north pole.

Interaction between Earth's Magnetic Field and Magnetic Moment of the Magnet:
When a thin long piece of magnet is suspended freely, it aligns itself with the Earth's magnetic field. The north pole of the magnet points towards the magnetic north pole of the Earth, and the south pole of the magnet points towards the magnetic south pole of the Earth. This happens because of the interaction between the Earth's magnetic field and the magnetic moment of the magnet.

Conclusion:
Hence, we can say that a thin long piece of magnet comes to rest in the North-South direction when suspended freely due to the interaction between the Earth's magnetic field and the magnetic moment of the magnet.