A grinder including a hollow base, a receptacle, an automatic open/close control device, a grinding unit, and a housing. The automatic open/close control device has a follower member, a bottom cover, a position-limitation structure, and a torsion spring connected between the follower member and the bottom cover. When a friction force produced during grinding of the grinder is greater than a torsional force of a torsion spring, the follower member is rotated relative to the bottom cover from a first position where radial blocks of the follower member block outlets of the bottom cover to a second position where the radial blocks of the follower member are moved away from the outlets of the bottom cover and guide slots of the follower member are moved into alignment with the outlets respectively for discharge of ground powder out of the grinder through the guide slots and the outlets.