fall open (verb)

1. As the suspenseful scene unfolded, the protagonist's eyes widened in shock, causing their mouth to fall open.

2. Upon witnessing a divine miracle, the congregation's mouths fell open in awe and reverence.

3. The grand entrance of the palace featured a set of intricate double doors that would fall open upon the arrival of important guests.

4. The protagonist cautiously opened the old book, causing it to fall open, revealing a dark and sinister spell.