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Thought-cubes were devices shaped like small green cubes. They were used by the Zilgors as their equivalent of books and were accessed by telepathy. The Zilgors brought several thought-cubes with them to Bruhl.
The Glacian scientist Ffreuf, who opposed the tyrannical reign of Rraprro, stole a thought-cube to learn why the Zilgors had left, but was unable to discover how to use it. Grahm was accused of stealing it and was pursued by the police. Ffreuf freed the First Doctor and Grahm from prison in the hope that the Doctor could help discover how to use it. The Doctor learned from the thought-cube that the Zilgors had been actually been frozen in ice and hidden in a cave. (PROSE: Justice of the Glacians)