Why do some lizards have a third eye on top of their head?

The so-called third eye in some reptiles is called the parietal eye, and it mainly senses light rather than forming detailed images. It can help regulate circadian and seasonal behavior by detecting changes in daylight from above. This is useful for basking, timing activity, and possibly noticing aerial predators. The structure is especially well known in some lizards and the tuatara, a reptile from New Zealand. It is a reminder that evolution can preserve unusual sensory systems that serve a specific purpose.