How can i crush oyster shells to add to my chicken feed?

ALL YOU HAVE TO DO IS MAKE HARD BOILED EGGS. U EAT THE INSIDES. AND CRUSH THE SHELLS WITH THE BOTTOM OF A CUP.NO NEED FOR OYSTER SHELLS. GREEN FOODS HAVE CALCIUM IN THEM. LET THEM OUTSIDE THEY WILL EAT THE SMALL ROCKS AND GET THERE CALCIUM!!