What is it called when you stare with your mouth open?

There isn't one specific word for staring with your mouth open. However, there are several expressions that capture the idea:

General:

* Staring blankly

* Gazing with an open mouth

* Mouth agape

* Open-mouthed

* Dumbstruck

* Astonished

More specific:

* Gaping (implies surprise or shock)

* Slack-jawed (implies astonishment or bewilderment)

* Wide-eyed and open-mouthed (emphasizes surprise and wonder)

The best word to use will depend on the specific context and the emotion you want to convey.