Let's say you're in a hallway, and you're walking, and another person is coming towards you, and it'll be another 20-30 steps before you pass each other.
How do you acknowledge the fact that you've seen them without it being weird, especially if that person is a stranger or an acquaintance and not someone you're really friendly with?
Do you make make eye contact with that person and not let go until you do pass by each other? Do you avoid eye contact until you're really close? Do you pretend to be lost in thought and avoid all eye contact? And if you smile or nod from the distance, at what point can you look away without seeming weird/rude?