How can a magnetised piece of iron be demagnetised?

Two identical looking iron bars A and B are given, one of which definitely known to be magnetised. How would one ascertain, whether or not both are magnetised? If only one is magnetised, how does one ascertain which one? Use nothing else but the two bars A and B.

An unmagnetised piece of iron is attracted to a bar magnet. Explain the origin of this attractive force.

An iron bar is magnetised with the help of another magnet or by subjecting it to a magnetising field. The magnetism acquired by the magnet is assumed due to the alignment of molecular magnet. Does the length to the iron bar undergo a change during the magnetisation process?

Why are all pieces of iron not magnet even though iron is a ferromagnetic material?

How are materials classified according to their behaviour in a magnetic field? Why does the magnetisation of a paramagnetic salt increase on cooling?

If temperature and magnetic field applied across a paramagnetic substane are trippled, how many times intensity of magnetisation of the substance will change?

Why ordinarily a piece of iron does not behave as a magnet?

Answer the following questions: Would the maximum possible magnetisation of a paramagnetic sample be of the same order of magnitude as the magnetisation of a ferromagnet?