drop open
fall open

His mouth dropped open at what he saw directly below.

My mouth dropped open for a second, but I said nothing.

His mouth dropped open as she shut the door behind her.

All eyes turned and almost immediately every man's mouth dropped open.

I was trying hard to keep my mouth from dropping open.

His mouth dropped open to speak, but he closed it without saying a word.

Instead, her mouth dropped open as she read the name of the judge sitting on the case.

When he saw what was inside, his mouth dropped open.

With an effort he kept his mouth from dropping open.

Then, he got a good look at the boy, and his mouth dropped open.