And something similar from everyday life.
One day the speed of bypassing car might be measured with a ruler and a clock. The next day, if the same measurement gave a different answer, no one could tell if the speed of the car changed, the ruler length changed, or the clock ticking changed, or your car is moving differently, or your eyes became older.

So what is here relativity for?

>One day the speed of light might be measured with a ruler and a clock. The next day, if the same measurement gave a different answer, no one could tell if the speed of light changed, the ruler length changed, or the clock ticking changed.