Mentos

 was a computer that had all the knowledge of the universe. It was set to answer all the questions posed at it until it got one wrong. It was needed to save the Generios system from destruction as one of the system's three great treasures, which would be paid as 'tribute' to the mysterious Cylinder. The Sixth Doctor asked it two questions he thought it would never answer, the colour of the wallpaper at Professor Watkins' house and the three wishes he made on his 900th birthday. Sally-Anne Stubbins found the right question, "What does Mentos not know?" The paradoxical question stumped Mentos. This allowed the Doctor to deactivate him and take it away. (AUDIO: The One Doctor)