Imagine two
twins, one of whom stays at home, and the other
travels out into space and back, reaching speeds
close to the speed of light.
Depending on
how close to the speed of light the traveling twin
comes, less time will have passed for him on his
trip than for the twin who stayed at home.
The effect is
not symmetrical, and all observers can distinguish
between the twin who traveled and the one who
didnŐt.