One word for brief interactions with strangers

Is there a single word for a brief interaction with a stranger, for example, when someone offers you their seat on the bus or holds the elevator for you?

Edit: The interaction needn't necessarily be polite or courteous. It might even just be somebody tripping over their own two feet and bumping up against you at the bar.

encounter (noun):

an unexpected or casual meeting with someone or something

'When you go to work, or visit a park, it is possible that you will have a range of unexpected encounters, however fleeting or seemingly inconsequential.'

Other suggestions include:

meeting (noun):

(Defn. 2) a situation when two or people meet, by chance or arrangement

'While there, I've met people I already knew by chance and had chance meetings with new people.'

You would probably need to refer to 'chance meetings', or 'meeting by chance'.

Source: ODO

Another good word would be "have a brush with":

have a brush with something

to have a brief contact with something; to have an experience with something. (Especially with the law. Sometimes a close brush.) Ann had a close brush with the law. She was nearly arrested for speeding. When I was younger, I hada brush with scarlet fever, but I got over it.