Greetings! They come in various flavours, depending on how well you know the person you're greeting and the time since the last greeting. I came across a funny one today.
In descending order of warmness, there's the hug, the informal "hi!" or "hey!", the slight nod of the head, and then there's the funny smile. It's not a real smile. The real smile is reserved for people you can't say hi to because they're on the phone or for people you're flirting with.
The funny smile is the kind of smile where you consciously try to move the corners of your mouth a little bit upwards while keeping the lips pressed together. The meaning is something like, "I see and acknowledge you but a real greeting is not warranted".
The funny thing about it is the way it looks. I haven't thought about it before, but someone sent me such a funny smile while I was on my bicycle to work. And you surely appear to be sucking your lips into your mouth, like the archetypal toothless old witch.
Think about that next time you funny-smile someone.