Why does my bearded dragon sit there with his mouth open?

This is normal behavior for bearded dragons. Some owners frequently fail to provide the correct temperatures within their bearded dragon enclosure, so they don’t see this behaviour very often. The key to keeping bearded dragons happy is to have a basking spotlight that allows them to raise their body temperature to 35-43.3°C (95-110°F). When a bearded dragon reaches its thermal maximum, it will often sit with its mouth open. This behaviour, called gaping, shows that the lizard is at its optimal temperature for basking. This gaping will allow a bearded dragon to dissipate extra body heat. Since lizards don’t sweat, this is an effective way for them to regulate their body temperature. It is important that your bearded dragon habitat have a thermal gradient, so that they can move to a cooler location once they have reached their optimum body temperatures.