I didn't believe it either, but several months ago I had it explained to me in a manner I could follow.
As strange as it seems to someone who, like me, never advanced beyond high-school physics, matter-to-energy conversion and vice-versa does apply to chemical and mechanical reactions. Just to so slight a degree that it is not worth bothering with for explaining simple quotidian events.
Question #2: Why does the Hubble redshift expansion appear to give the age of the universe as 1 billion years when the earth's surface is 4 billion years old and the Big Bang apparently happened 14 billion years ago?
Question #3: Why did Einstein assume that nothing moved faster than the speed of light and then conclude after a shower of tensors that nothing can move faster than the speed of light?