Why does my Bearded Dragon sit with their mouth open?

Beardies (and other lizards) often bask with their mouth open and it's like the lizard equivalent of a dog panting in the heat. It's completely normal.

However, if your beardie lunges at the glass with its mouth open when you approach, without actually seeing it, I'd say it can either be through fear (possible in juveniles not used to handling or which have had more than one home) or possibly just an excited feeding response as it knows you might be about to give it some tasty bugs!

If the latter, i would ensure the beardie always has had its salad/fresh leaves early in the morning and that bugs are not fed until later in the afternoon once the Viv is nice and hot (3–5pm?!) and only if it hasn't eaten a good proportion of its' greens first!

Some owners complain their bearded dragon doesn't eat/like salad and this is usually either because they feed waaay to many bugs or feed them at the wrong time (before the salad has been eaten!) or that the enclosure is too cool (correct temperature for the COOLER end is 85°F/30°C, the hotter end/basking spot must be a MINIMUM of around 110°F/45°C but can be warmer as long as the cooler end remains correct).

Beardies should get most of their moisture from their leafy greens (most leafy salad leaves are fine other than kale or spinach which can prevent the correct absorption of calcium - particularly in younger lizards) so it's important for them to eat those first.

A beardie will always eat bugs, originating from a desert environment where food is scarce, their instinct is always to eat when food presents itself, hence why adult beardies are often overweight as their owners feed too many bugs:salad ratio.

Ideally the enclosure should be situated in a busy environment such as a living room, kitchen etc where there is lots of daytime activity for the lizard to see what's going on and get used to people walking near/past its cage without always expecting to come out. Some owners with beardies in upstairs bedrooms get this behaviour as the bedroom is often unoccupied/boring (from the lizards perspective) during the day and then when a person enters the room, the lizard gets excited as often this means food! It can also be a reason why lizards in this situation could be more prone to being jumpy/aggressive (ie scared).

Some owners find taking the lizard out of its fate and feeding it some bugs whilst it's walking around on the floor (or maybe in an empty bath) allows the lizard to run around and chase ('hunt') the insects which is both fun to watch but rewarding for the dragon & gives it some exercise!