The person in the flat below you will age more slowly than you

One curious consequence of the theory of relativity was time dilation, which means that time moves slower the closer one is to a massive object. Although this effect is small on Earth, it is noticeable. "If you live in a two-story house on the top floor, you will age faster than your neighbor below by about 10 nanoseconds in one year," said quantum physicist Igor Pikovski at the Harvard-Smithsonian Center for Astrophysics. "This tiny effect has actually been confirmed in many experiments with very precise clocks.