In the passage, the question “Why does the metal respond in this way?” ----.

Why do transformers hum? This is one of those
questions which seems easy but has surprising
hidden depths. The simple explanation is that
electric currents create magnetic fields, and the
alternating current of mains electricity used by
transformers creates a magnetic field that changes
at 50 cycles a second. This in turn triggers a regular
motion of the metal molecules inside the
transformer, known as magnetostriction. It's this
motion that makes the surrounding air vibrate,
creating the hum. But why does the metal respond
in this way? The answer lies in a property of the
electrons in the metal known as ‘spin' - a property
which can only be explained by reference to
Einstein's theory of relativity, which is beyond the
understanding of most of us.