It's not an illusion—there really are tiny sparks of light inside your eyeballs. In fact, the cells in your eyes are constantly emitting light called biophotons. That’s because, like everything, your eyes are made of atoms, and atoms contain elementary particles that transmit light. Typically, your brain simply ignores these distracting flashes, but when you rub your eyes, stand up too fast, or sneeze hard, the pressure creates too many biophotons for your brain to handle, and they become visible for a brief second. Technically, these are called phosphenes, but we know this occurrence as seeing stars.