Air that has been pushed into the vagina by the {penis, dildo, fingers...} during sex often re-emerges with a fart-like sound. Certain people consider it foul though it is only air.

How you react is very important, because the person who just did it is probably embarrassed and insecure about it.
Here are two helpful ways to react to a queef:

1) ignore it as if it is normal or
2)smile and chuckle (together) about how interesting our bodies are.
If you do not know the person well it is best to somehow indicate that you are not offended. Which option is chosen depends on your history with the person and the general mood of the lovemaking.

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