Consider four (dimensionless) flies, two males and two females. They are situated at the corners of one square meter. Every fly tries to reach the male/female fly in front of her/him. Their initial situation is visualized in the picture below. Since the flies are flying towards another, they will meet each other at a certain time in the center of the square.

The question: What is the length of the path they have traveled at the moment they reach each other?

The answer: Click here!