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When people want to show they are kidding or have just knowingly uttered a falsehood, they stick their tongues in their cheeks, so it's "tongue in cheek," not "tongue and cheek."
This funny-sounding expression indicates that someone just told a joke or isn't being serious! If someone says something "tongue-in-cheek," he or she is usually kidding.
It's believed that this saying was created by an English humorist in the 1800s. Most people have difficulty saying anything with their tongue in their cheek. But some people actually do stick their tongue against the inside of their cheek after saying a joke to show that they're only kidding.
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