When someone lets you into the flow of traffic, or holds a door, or takes a second to acknowledge you, it's possible to smile and offer a wave in response.
This, of course, costs you nothing.
It creates a feeling of connection, which is valuable.
It makes it more likely that people will treat someone else well in the future.
And it might just brighten your day.
The simplest antidote to a tough day is generosity. Waves are free, and smiles are an irresistible bonus.