In order to understand how he did we need to understand the Doppler effect.  That is explained in the following video.

Another thing we need to know is that the speed of light that we measure is always the same no matter how fast we are moving.  This is a very strange fact.  Imagine you are standing still and someone throws a ball at you at 20 miles per hour.  Then image the same person runs towards you at 10 miles per hou and throws the ball toward you at 20 miles per hour.  It will  hit you harder the second time because he was moving toward you when he threw the ball and so it gets to you faster.  It may have left him at 20 mph but it approaches you at 20 + 10 = 30 miles per hour.  The wierd thing about light is that if he is holding a flashlight and aims the light at you it won't move toward you at a different speed no matter whether he is stationary or moving toward you or moving away from you.  There are many strange conclusions that can be drawn from this. One conclusion that Einstein drew was that there is an enormous amount of energy in mass.  How he drew that conclusion is explained in the video below.

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