What do you do when you make eye contact with a stranger?

Well, let’s say you’re walking down a hallway at school, or even down an aisle at the supermarket. You make eye contact with a stranger (either they looked at you first, or you looked at them first), or maybe somebody that you’ve seen around before, but you’ve never spoke to them, what do you do? Smile? Look away? Ignore them? Wave?

I’m just curious as to what people would do in this situation. I guess this may sound extremely strange, but I don’t know what I should do when I make eye contact with strangers, or acquaintances. It seems the most obvious thing to do is smile, but I find it hard to smile at people when I don’t know them, and also if neither of us is about to say a greeting of some sort.

I usually just ignore and look away, as if they’re nothing. But this, I just realized, seems kind of rude? Am I being rude? I see them, but then my eyes roll onto something else. What SHOULD I be doing?