drop open
fall open

My mother's mouth fell open and she held out her hands.

Her mouth fell open when she saw what they were.

What could we do when the border suddenly fell open?

From the corner of my eye I saw her mouth fall open.

I turned to look at him for the first time, and tried not to let my mouth fall open.

Her mouth fell open but no sound would come out.

His mouth fell open, but the words were a few seconds behind.

He actually felt his mouth fall open with the shock.

What he saw in the next instant made his mouth fall open.

Her mouth fell open, she had no idea in what shape.