One twin flies near light-speed and returns younger than the one who stayed
Special relativity says a moving clock runs slow. But from the traveling twin's point of view, it's the Earth twin who was moving — so shouldn't the Earth twin be the one who ages less instead? Two observers, each with equal claim that the other's clock should be the slow one: which twin actually ages less when they're finally reunited?
Reveal the answer
The traveling twin ages less, and the apparent symmetry is only that: apparent. Unlike the twin who stays on Earth in a single unchanging frame the whole time, the traveler has to accelerate to turn around and come home, which breaks the symmetry between their two points of view. Paul Langevin popularized the puzzle in 1911 to illustrate special relativity, and the effect has since been measured directly using precise atomic clocks flown on jet aircraft.