When is a principal vicariously liable for torts committed by its agent?

Hypo 1: Amy hires Bob at her bakery. One day, Amy realizes she is out of sugar. She tells Bob to go to the store and buy more. Bob gets in his car and, on the way to the grocery store, crashes his car into Sam. Result: Bob crashed his car while running a business-related errand for Amy, so Sam could sue Amy for his damages under respondeat superior, a fancy Latin phrase for vicarious liability that means "let the master answer."

Hypo 2: Amy hires Bob at her bakery. One day, Amy realizes she is out of sugar. She tells Bob to go to the store and buy more. Bob gets in his car and, on the way to the grocery store, decides he wants to visit a strip club. As he's pulling into the strip club parking lot, he hits Sam's car. Result: Bob may have initially been running a business-related errand for Amy, but his personal decision to deviate from that plan and visit a strip club severs the link to his employer. As a result, Sam may not sue Amy for the damage.

Hypo 3: Amy hires Bob at her bakery. One day, Amy realizes she is out of sugar. She tells Bob to go to the store and buy more. Bob gets in his car and, on the way to the grocery store, decides he needs to get gas. As Bob pulls into the gas station, he hits Sam's car. Result: Bob crashed his car while running a business-related errand for Amy. Even though he didn't go directly to the store, the brief detour to get gas does not eliminate a link to his employer. Therefore, Sam may sue Amy for his damages.