Movie question please read to help me figure it out. ASAP?

Ok so i saw this small clip from Witch Yoo hee, some asian drama but they were watching an american movie. The movie showed this boy leanedd over drinking out of a water fountain, an it is a real calm clip and all of a sudden al lyou see is some girl slip her tounge into his mouth an he jerks back. an apparently this happened when they were younger. I suppose at the beginning it was more like a garden type deal a large mansion an they first meet as kids. It seems like a serious movie. I jsu cant remember the name. i have only seen clips si u cant give any more details. PLEASE HELP! Driving me nutts!