Twin Paradox

The twin paradox is both fascinating and tragic. The experiment where one twin is on a ship that is going close to light speed and the other is on earth and they both send the other letters once a week. As the distance grows it will take longer and longer for the twins to receive the other letters thanks to spatial relativity. The twin on the ship will eventually return home. They will begin to receive the other twin's letters more and more frequently. They will read about them growing old and eventually dying. When they get back to earth it would be so far in the future the grandkids of the twin on would be dead by now.

This is because of how special relativity works, effectively slowing the aging of the twin on the ship while the aging of the other stays the same. So, the twin on the ship basically time-traveled to the future.