A rabbi gathered his students together and asked them: “How do we know the exact moment when night ends and day begins?”

“It’s when, standing some way away, you can tell a sheep from a dog,” said one student.

The rabbi was not content with the answer. 

Another student said: “No, it’s when, standing some way away, you can tell an olive tree from a fig tree.” 

“No, that’s not a good definition either.”

 “Well, what’s the right answer?” asked the students.

And the rabbi said: “When a stranger approaches, and we think he is our brother, that is the moment when night ends and day begins.”