Reaction expected when one of world's biggest stars crashes your wedding

Katy Perry knows how to start a dance party!

The "Swish Swish" singer hit the dance floor with the bride, after she crashed a couple's wedding at the Four Seasons Hotel in St. Louis. 

Perry wasn't quite dressed for the tuxedos and formal evening gowns event. But that didn't stop the guests from wanting to grab selfies with her.