A facultative biped is an animal that is capable of walking or running on two legs, often for only a limited period of time, in spite of normally walking or running on four limbs or more. Well known examples include many lizards such as the Basilisk lizard, and even some cockroaches when running at top speed. In order to be considered a true facultative biped, an animal must be capable of sustained movement over many strides while bipedal - simply adopting a static bipedal posture while resting or looking around is not sufficient.