I never understood why speed of light is a constant (c)... until now!
inv.nadeko.net
Why is the speed of light the same in all reference frames? Let's rediscover the thought experiments that led Einstein to his special theory of relativity Chapters: 00:00 Introduction 01:15 1/3 Detect motion with particles? (Thought experiment) 04:01 Inertia doesn't allow detecting constant velocity motion 06:18 2/3 Detect motion with waves? (Thought experiment) 09:18 Medium doesn't allow detecting constant velocity motion 10:15 Constant velocity motion is RELATIVE! 11:02 3/3 Detect motion with light? (Thought experiment) 13:12 Does light break relativity? 13:46 Michelson & Morley's experiment (oversimplified) 14:20 The logical conclusion - Speed of light is same in all frames 15:32 Discovering time dilation & length contraction 19:59 Summary

This video is a bit older now (over one year), but I just found about it. And I want to share it here, because it was very well explained, without boring stock videos or background music. It’s just like a teacher would teach you, but with some enthusiasm behind it. I enjoy his videos so far.

Video description:

Why is the speed of light the same in all reference frames? Let’s rediscover the thought experiments that led Einstein to his special theory of relativity

Chapters:

00:00 Introduction
01:15 1/3 Detect motion with particles? (Thought experiment)
04:01 Inertia doesn't allow detecting constant velocity motion
06:18  2/3 Detect motion with waves? (Thought experiment) 
09:18 Medium doesn't allow detecting constant velocity motion
10:15 Constant velocity motion is RELATIVE!
11:02 3/3 Detect motion with light? (Thought experiment)
13:12 Does light break relativity? 
13:46 Michelson & Morley's experiment (oversimplified) 
14:20 The logical conclusion - Speed of light is same in all frames