The square sticks are coated with zinc sulfide, a compound of zinc (metal) and sulfur, over a length of approx. 3-4 cm. This zinc sulfide has the amazing property of storing light and gradually emitting it again in small portions. This can take minutes to hours. One speaks of luminescent dye, technical term: phosphorescence. All clock hands and dials that glow in the dark are coated with zinc sulfide. The shining stars in children's rooms as well as the fire brigade information signs in public buildings.
 
This is how it works: You hold the light stick near a light source for a few seconds; the stronger the light, the better the effect (optimally: LED light). After the light source has been extinguished, the stick glows greenish like a firefly - the darker the room, the brighter the glow. You can repeat this effect as often as you like, the zinc sulfide is not used up. A life long! It's also not poisonous or dangerous.