When a charged electroscope is touched with a metal rod, the leaves collapse because the charge on the electroscope is transferred to the metal rod. This happens because metals are good conductors of electricity, which means that electrons can easily flow through them. When the metal rod touches the electroscope, the electrons on the electroscope are transferred to the metal rod, leaving the electroscope with a neutral charge. Since the leaves of an electroscope only spread apart when there is a difference in charge between the leaves and the body of the electroscope, the leaves collapse when the charge is neutralized.