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 Galarias system from destruction. The Sixth Doctor asked it two questions it 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?" This allowed the Doctor to disassemble him. (AUDIO: The One Doctor)