Soniya, the scientific is quite simple.... Human brain can not focus on two fucntions that originate from two different sources. When people attempt to complete many tasks at one time and or rapidly between them, errors go way up and it takes far longer. The brain is forced to pause and refocus continuously as one switches between tasks. This is the reason why answering a phone call whilst driving is not recommended as human brain can not concentrate fully on two activities at the same time.