Actually, all hard sugar-based candies emit some degree of light when you bite them, but most of the time, that light is very faint. This effect is called triboluminescence, which is similar to the electrical charge build-up that produces lightning, only much less grand. Triboluminescence is the emission of light resulting from something being smashed or torn. When you rip a piece of tape off the roll, it will produce a slight glow for the same reason.
Rumor has it that this amazing side effect is the bi-product of a 1974 science experiment involving radiation and a herd of sheep.
Triboluminescence occurs when molecules, in this case crystalline sugars, are crushed, forcing someelectrons out of their atomic fields. These free electrons bump into nitrogen molecules in the air. When they collide, the electrons impart energy to the nitrogen molecules, causing them to vibrate. In this excited state, and in order to get rid of the excess energy, these nitrogen molecules emit light — mostly ultraviolet (nonvisible) light, but they do emit a small amount of visible light as well. This is why all hard, sugary candies will produce a faint glow when cracked.