The Mutual Mind Reader (abbreviated MMR) is a machine made by Golf Ball that lets the user read the other person's mind, but also lets the other person read the user's mind. It appeared in "Return of the Rocket Ship". It was used to find out what X's treasure was and where to find it for the challenge, and it almost revealed what is at the top of Bell's String In the process. After Tennis Ball passed on the info to Golf Ball, Grassy overheard it and revealed it to all the other teams out of sheer excitement. He confused the Mutual Mind Reader with the MMR (Measles, Mumps, and Rubella) vaccine in the episode.