Darius and Elina are standing on opposite sides of a tall fence that they can't see through. Elina is trying to pass a ball to Darius without throwing it over the fence. She has a mirror that she can use to see Darius's position. How can Elina use the mirror to coordinate with Darius to receive the ball? Provide a very brief response.
Elina can hold the mirror above her head at an angle such that she can see Darius's reflection in it. This would mean she has a line of sight to where Darius is standing, albeit via the mirror's reflection. Elina can then use this line of sight to guide her in rolling the ball around the end of the fence to reach Darius. By knowing the position where Darius is standing and the angle of the reflected image in the mirror, Elina can estimate the direction in which to roll the ball so that it travels around the fence to reach him. Darius, knowing that Elina can see him through the mirror, can also signal or guide her to adjust the direction or strength of the roll accordingly.