This will enable you to see sparks more easily. Lightning is a stream of electricity that excites nitrogen molecules in the air. Electricity provides excited nitrogen molecules with extra energy, which the nitrogen molecules then release as visible light. When you crush sugar crystals with your teeth, the pieces become positively and negatively charged. This makes the electricity jump through the air between the pieces of sugar, exciting nitrogen molecules and making them emit light.
Chemical energy is stored in all kinds of molecules.
With sugar crystals and other substances, this energy can be converted to light energy by using pressure. That's because a lot of the energy that gets released from plain old crushed sugar gets released as ultraviolet light, which the human eye can't see. This process is called fluorescence .