Time does not flow the same for everyone — a clock moving very fast or sitting near a huge mass runs slightly slower.

Picture space and time as a stretched elastic sheet. A heavy planet bends it, just like a heavy ball bends a stretched blanket. Other objects passing by slide along that curve, which we call gravity. The heavier the mass, the stronger the curve — and the stronger the effect on time.

Why it matters Without this correction, the GPS in your phone would be wrong by kilometers every day.