Final answer:
Shocks are more common on a dry day because dry air is a better insulator, allowing the charge to build up and create a larger potential difference between the person and the metal object.
Step-by-step explanation:
When walking on a rug, a person frequently becomes charged due to the rubbing between their shoes and the rug. This charge can cause a spark and a slight shock when the person gets close to a metal object. These shocks are more common on a dry day because dry air is a better insulator, allowing the charge to build up and create a larger potential difference between the person and the metal object.