Matter may be converted to <answer>energy</answer> (and vice versa), but mass cannot ever be destroyed; rather, mass/<answer>energy</answer> equivalence remains a constant for both the matter and the <answer>energy</answer>, during any process when they are converted into each other. However, since  is extremely large relative to ordinary human scales, the conversion of ordinary amount of matter (for example, 1 kg) to other forms of <answer>energy</answer> (such as heat, light, and other radiation) can liberate tremendous amounts of <answer>energy</answer> (~ joules = 21 megatons of TNT), as can be seen in nuclear reactors and nuclear weapons. Conversely, the mass equivalent of a unit of <answer>energy</answer> is minuscule, which is why a loss of <answer>energy</answer> (loss of mass) from most systems is difficult to measure by weight, unless the <answer>energy</answer> loss is very large. Examples of <answer>energy</answer> transformation into matter (i.e., kinetic <answer>energy</answer> into particles with rest mass) are found in high-<answer>energy</answer> nuclear physics.
Matter may be converted to what?
energy