Sorry if this sounds dumb, but in the famous equation e = mc2 why is the speed of light

I recently read E=mc2: A Biography of the World's Most Famous Equation by David Bodanis... thoroughly enjoyed it... but the answer to this question eludes me.

Can anyone help in

**squared**?I recently read E=mc2: A Biography of the World's Most Famous Equation by David Bodanis... thoroughly enjoyed it... but the answer to this question eludes me.

Can anyone help in

**layman's terms**please?
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