How History Speaks on Instagram: "What looks like a simple marble toy is actually a 700-year-old mathematical idea in motion. This device demonstrates the Tusi Couple, where two perfectly synchronized circular motions combine to create a straight-line path. Invented by 13th-century Persian scholar Nasir al-Din al-Tusi, this principle was once used to explain planetary motion in astronomy. The steel balls only appear to move straight, but their path is the result of pure geometry cancelling curve