A “flash mob” happens when a group of seemingly random strangers suddenly start dancing, singing or moving in exactly the same way.
For instance, a group of people in a mall may suddenly all start dancing to a Lady Gaga song, doing all the same moves.
The rest of the people in the mall, who are not part of the flash mob, get treated to a surprise show.
When the show is over, the people who were in the flash mob just walk away, as though nothing had happened.
There was a flash mob in Toronto on Sunday.