Ms. Frizzle and Liz have invented a magical pinball machine that uses light instead of balls. (The light travels much slower than in the real world.) The light goes through a prism and the player attempts to bounce the different colored beams of light into the appropriate eye, in order to "make the rainbow." Ms. Frizzle says if she fails to win the game, it will be taken away. The class decides to help, and they shrink the bus and go inside the machine. Mr. Ruhle, the principal, shows up, and Arnold tries to stall him.