Inez and Digit have the key to escape the cave, and they need to get it to Jackie and Matt in order to escape. There is a huge block of ice blocking the two groups, so they decide that bouncing the key against a wall is the only option. They get one chance to bounce the key or they can lose it, so Inez uses a drawer that Digit found to test the angle that would send the key to Jackie and Matt. During the process, they learn about how angles of reflection work.
Beakman explains how light rays reflect off of smooth vs. bumpy surfaces. Since a mirror is smooth and is coated with a metallic backing, the light comes right back at the perfect angle, creating a reflection.