Rats wandered a maze with no reward, then proved they'd mapped it all along

Edward Tolman let rats explore a maze for days with no food waiting at the end, and their performance barely improved. The moment he added a reward, they suddenly ran it almost as fast as rats rewarded from day one — revealing they had built a mental map of the maze all along, just with no reason to act on it until it paid off.

— Edward C. Tolman and C. H. Honzik, Introduction and Removal of Reward, and Maze Performance in Rats — University of California Publications in Psychology, 1930