The killer bicycle is Calvin's bicycle. It appears to be a normal bicycle, but when nobody but Calvin is around, it comes to life, similar to Hobbes, and attempts to maul Calvin. It is kept in the garage. Somehow, it is capable of ringing doorbells, as it did so in a Sunday strip in order to enter Calvin's house. One time, it hid in Calvin's closet, and tried to ambush him at night. Another time, it charged into the house and made a mess of tire marks. Hobbes one gave Calvin advice to defend himself from the bicycle, which miraculously worked. At one point, Calvin's parents put training wheels on the bike.