Magnesium will react with an acid and make hydrogen gas.
The more concentrated the acid, the faster the magnesium will react
and you will see bubbles of hydrogen being produced more quickly.

A strong acid and a weak acid of the same concentration
will react at different rates with the same metal.

You can see the difference between a strong acid and a weak acid
of the same concentration by looking at the reaction with magnesium.
The strong acid reacts faster and you see more bubbles of hydrogen.
This is because the strong acid has more hydrogen ions in the solution
even though it is at the same concentration as the weak acid.
A piece of magnesium the same size
should be used in both acids for the comparison to be fair.