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.