These plastic necklaces are hollow. The tube is filled with non-toxic chemicals that undergo a slow reaction called chemi-luminescence. This reaction creates light, not heat. To start the reaction, you have to bend the tube. By doing so, a miniature glass vial is broken inside the tube, allowing two chemical solutions to mix and give off energy. These solutions are like fuel, and pass through a fluorescent dye, which is also present in the tube. The necklace glows until the "fuel" runs out.