Why did Einstein use "c-squared" in his famous equation? I understand energy equals the speed of light squared, but what does the speed of light have to do with it and why square this quantity?

Dear spatz406

That's easy to ask, but difficult to explain in 3 or 4 lines of text.
First of all : energy is Mass times the speed of light squared.
Also : energy is not something you just store in a box or jar.
When you describe energy in an algebraic way, it has already the speed of light as one of it's components.
When conversion of mass takes place into energy, this speed of light is multiplied once more with the speed of light , and hence we write that as the square of the speed of light.

The energy released in a car crash is directly related to the mass and the speed involved.
Linear in mass, but squared in speed.
If you hit a car against a solid wall, the damage following a crash is 25 times bigger, if the speed is 5 times higher.
That one you can even try out yourself, though I suggest you take my word for it !

:)