ELI5 Einstein - why is e = mc squared such a big deal?

Why is the discovery of e=mc^2 such a big deal? How important was it really? In what way did it make smartphones possible? What was special about the way Einstein was taught algebra? Why do we always think of the iconic photo of Einstein with his tongue out?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: cumoisseur, grungyit, rasye and polar_roid

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