Description of the demonstration
At first copper rod will be deformed by bending. At this stage, the volunteer will not need to apply too much forces. Metal will bending easily.
After then another volunteer will try to straighten out the bent rod, to return it to the original shape. You will see that the force required for this much more.
This is happening because of dislocations are created and they intersect with another ones. Therefore their mobility decreases and deformation becomes difficult.