Not returning courtesies to people is considered rude.
Rick and Ron are coworkers who are passing each other in the hall on the way to their offices, and Ron nods to Rick as they walk by.
Rick needs to decide how to respond.
Rick gives a silent nod in return as he passes Ron in the hallway.
Rick does not return the nod, pretending to be lost in his thoughts as he walks by.
Ron is satisfied that his greeting has been acknowledged.
Ron is annoyed at not having his nod returned in kind.