The GPS system requires Einstein's theory of relativity to work.
Why it is true
Because satellites move very fast and are further from the Earth's centre of gravity, their atomic clocks tick at a different rate than clocks on the ground. Without Einstein's corrections, GPS would fail by kilometres a day.
Source: ESA
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.