What is the difference between a frolic and a detour?

Imagine you asked three different employees—Amy, Bob, and Sam—to run errands for your business. You ask Amy to go get coffee for the office, Bob to go pick up a box of office supplies, and Sam to pick up your suit from the cleaners. All three of employees end up getting into accidents. Amy got into an accident on her way to the coffee shop, Bob got in an accident on his way to get gas, and Sam got in an accident on his way to visit his girlfriend who lives near the cleaners.

You are liable for Amy's accident, because she was on her way to the place you told her to go. You are also liable for Bob's accident, because him filling up his gas tank would be merely a detour in the ultimate pursuit of accomplishing his errand. However, you are arguably not liable for Sam' accident, since his decision to first travel to his girlfriend is a completely independent journey that has nothing to do with his assigned task, otherwise known as a frolic.