Thrown together in the air, they are attracted by the magnets inside the stones, and "sing" when they meet.
Length of the stones about 4.5 cm.
Tip: try to throw the stones into the air with some air between them. In flight, the magnets attract each other, collide and develop the rattle noise. It takes a bit of practice to get the hang of it :-) Magnetic stones, singing magnets, rattle snake, magnetic rattle
ca. 1-2 days (abroad may vary)