Taylor Swift joins HAIM for a surprise performance

After this surprise, fans can't stop laughing. 

she surprised attendees at HAIM London's July 21 show. 

Taylor's idea of collaboration during the concert was part of a master plan for getting the crowd 

"sing the loudest they have ever sung all night." 

Taylor followed HAIM's lead for her outfit, but the girls weren't just collaborating on music. 

"I must see that." It seems like about 20,000 people thought the same thing. 

Fans would be happy to copy and paste this concert experience over and over again, it's obvious.