On 24 August 2006, the International Astronomical Union voted to reclassify Pluto as a dwarf planet, ending its 76-year tenure as the ninth planet in our solar system. The decision sparked global debate and revealed that science had never formally defined what a planet actually was. Meanwhile, on the same date in 394 CE, a priest at the Temple of Isis carved what would become the last known hieroglyphic inscription, closing three millennia of Egyptian tradition. In 2000, Finnish chemists persuaded argon, a famously unreactive noble gas, to form its only known stable compound at temperatures near absolute zero. And in 2019, Nauru’s sitting president lost his parliamentary seat in a general election, immediately ending his presidency. Four moments when definitions shifted, traditions ended, and certainty proved more fragile than expected.