These lions, all being cuddled, display an open-mouthed response similar to saying "Aww!"

All three male lions here display a grin.  All of them have a very good reason to smile, as you might guess.  Note the lioness in the left photo is returning his gesture with an open-mouthed response.

This is called "Flehmen."  Not a "yuck," it's a reaction to strange smells that lions of both sexes show, allowing the Jacobson's organ in the roof of the mouth to "taste" smells.  Not even Einstein had a jacobson's organ...humans don't engage in flehmen.

The lioness on the left is giving you an intent look that could signal any sort of approach, from stalking prey to preparing for a play.  In the center is a typical calm face.  The lion on the right is panting.

No doubt about it, the first two have the look that says "Buzz off!"  The male in the center is more confident...his ears remain forward.  Hers go back.  The female in the third is even more defensive.    Humans, like lions, make a bite face, though they usually don't fight with their teeth.

The lioness on the left is stretching.  She shuts her eyes and the corners of her mouth turn down.  The male in the center is yawning.  The male on the right is roaring.  The tongue does not stick out while roaring.   Humans don't stick their tongue out when yawning or roaring...

These lions are both cringing.  It's a reaction similar to what you might do if someone sneaked up on you in a quiet library and grabbed you from behind.  This man was just grabbed by the tax auditor.

The cause of the first two expressions is a bit racy.  Let's say it roughly translates, "YESSS!!"  The third is good old fashioned grooming, but it makes my tongue cramp just watching it!   The man, like the lion, opens his mouth in a moment of extreme pleasure, but cannot lick his own neck.