In a disc grinder, a spindle mounted with a grindstone intersects an output shaft of an electric motor. The intermetallic compound is capable of occludingdesorbing lithium and contains at least one kind of element A selected from Sn, In, Ge, Ga, Pb, Al, Sb, and Si, and an element X that does not substantially react with Li. Each variable resistance layer may have an associated high resistance state and an associated low resistance state. The achromatic diffractive optical element can have a stepped multi-level profile with three or more levels, or a continuous profile. The heart-shape body include two lobes that direct movement, whether being used with one or two hands or with one user, partners, or in a group.