Here are some ways to think about it:
* Not literal: You're not actually putting your tongue in your cheek, of course. It's just an expression.
* Humorous intent: The goal is usually to make someone laugh or to highlight a humorous contradiction.
* Double meaning: The words themselves might be serious, but the context makes it clear that you're joking.
Examples:
* "I'm so excited to go to work tomorrow! I love my job!" (Said with a bored expression)
* "Yeah, I'm sure that's exactly what happened!" (Said with a raised eyebrow)
* "This weather is absolutely perfect for a picnic, wouldn't you agree?" (Said during a blizzard)
In each case, the speaker is being sarcastic and doesn't actually mean what they're saying. They're using a playful tone to convey their true feelings, which are the opposite of what they're expressing.