Turtle grinding

This morning, I was awakened by the sound of "shasha". What made this strange noise? I quickly got out of bed, take a closer look, hehe! These are the two turtles. I saw that the big tortoise continually grinds the armor on the sand. With the sound of "shasha...", a thin layer of gray powder appeared on the ground.

strange! Why do turtles wear their own shells? Doesn't it hurt? I asked my grandmother why, my grandmother told me: "That's a hard shell. It's uncomfortable. It's going to be worn." The book also said that snake skin, clam shell, turtle grinding is not the same reason?