The highly unrealistic love at first sight 'look' that is prominent in most rom-coms, love dramas, and pops up in other films too.
The girl walks past, the guy stops and stares with his tounge out in a trance like state - cue cheesy music, and some hair flicking and pouting from the chick - slow motion is often used as the guy pants like a dog and stares like a crazed psycho with mental health issues.
Nobody in reality does this, at all - A very over used cliche that doesn't represent the emotions, or actions, of an actual human being.