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Matter can definitely be created without violating conservation of energy. All massive particles (e.g electrons, quarks, etc) have a minimum energy called their rest energy, which is given by $E=mc^2$. This energy is stored as the mass of the particle. A particle can be created as long as there is energy at least equal to its rest energy, so that energy is ...


Please note that light is not a form of energy, rather it carries energy, just as matter does. In my opinion, the most clear and unambiguous example of "stuff" that is not matter is in fact light. One might regard spacetime also as a "stuff", a physical entity, but that is a bit more muddled in how you define "physical" and which model you are using. ...


It is fair for the middle school level. Mass-energy equivalence and the complications of space-time would make your lesson a bit more confusing so your statement should be good.

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