The red shift.
For students studying physics at least three:
If the wave source is removed, the wavelength is increased, if it approaches, it is reduced. What is logical: since the moment of emitting the previous wave, the source has moved. A fast approaching train whispers more than a fast departing train. From the swimmer who creeps to us, the waves that rush to the shore come more often - he catches them a little, from the floating - less often. Accordingly, the distant galaxies shine more "red," if they were approaching, they would shine more "violet."

For full humanitarian:
Get up at night on the road. The cars approaching light white lamps, the cars going away light red. What is incomprehensible?