A lock, including an outer subassembly, an inner subassembly, a safety mechanism, and a clutch. The clutch includes a square transmission tube and a sub-clutch. The safety mechanism includes a brake rod and a drive device. The inner subassembly includes an inner baffle plate, an inner baffle plate cover, a cylindrical tube, a transmission frame, and an inner doorknob. The inner baffle plate is inserted in the inner baffle plate cover. The cylindrical tube fits rotations of the inner baffle plate and the inner baffle plate cover. The transmission frame is disposed at one end of the cylindrical tube, and the inner doorknob is disposed at the other end of the cylindrical tube. The drive device includes a transmission member, a drive member, a first spring, a second spring, and a positioning slot. The positioning slot is disposed on the cylindrical tube.