The Twin Paradox

One twin stays on Earth. The other boards a spacecraft traveling at 90% light speed to a star 10 light-years away and back.

For the Earth twin, about 22 years pass. For the traveling twin, whose clock runs slow, the round trip takes only about 9.7 years. When they reunite, the traveler is biologically younger — same twins, different ages.

This isn't truly a paradox once you see the asymmetry. The traveling twin accelerated, turned around, and decelerated; the Earth twin didn't. That acceleration breaks the symmetry, and the twin who accelerated ages less.