Billy Keefe decided to celebrate her birthday in an unusual way.

Arlo Paden, 1, left, grins at Brett Stoppel, Lawrence, as they participate in a group hug Thursday evening in the middle of a crosswalk on Massachusetts Street. Billy Keefe, of Lawrence, organized the flash mob and asked her friends to wear red and come to South Park at 6:15 p.m. as a celebration of her birthday. Flash mobs, which started in New York in 2003, involve a group of people gathering in a specific location to do a specific act for a specific length of time. Flash mobs are usually organized on the Internet.