Walk up to someone in a crowd and sign happy birthday, wait and see how many people join in.
At lunch start singing the happy birthday song with you are one or two of your friends and see how many people you don't even know join in.
Sit on a bus, then stomp twice and clap. Everyone else will join in. This only works in America.
Go to a busy street and put on the music to YMCA and start doing it, see how nanny people join in.
Perform a silent act (miming) in front of every security camera you see. Get friends to join in!
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