There goes the first Law of Thermodynamics.
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Yeah...I thought matter to energy was a one-way street...no converting it back.
Photons can be converted to matter. It is called "pair production" or a "Compton pair." The total mass of the particles produced must be less than the E/c2, where E is the energy of the photon.
One of the things that is a consequence of Einstein's famous equation is that "mass-energy" is conserved; mass and energy separately are not conserved, at least under the general conditions of the universe. Under conditions of ordinary life, deviations from the "conservation of mass" and "conservation of energy" are so minute that they are not noticeable.
Most of the mass of anything is not mass due to the presence of matter, it is the mass of the energy that binds together the quarks within the protons and neutrons in every atomic nucleus of which the object is composed. That's something like 99% of the apparent mass of any object, including yourself.