Time Out says
If you’ve ever wandered through Berlin’s Mauerpark you may already be familiar with Bearpit Karaoke. If not, there are just four simple steps. One: Coax a random stranger into singing karaoke in the park. Two: See if a crowd gather to watch. Three: if things heat up grab another person to join in the sing-a-long. Four: Wash, rinse and repeat until you've got a fully-fledged guerrilla choir on your hands. Happy singing!