They say you're never too old to have your mouth washed out with soap. It seems Dave Grohl wanted to test that theory while appearing on stage with his mother. The pair gave a talk in Toronto on Sunday, discussing Virginia Hanlon Grohl's book From Cradle To Stage, which shares moments from her famous son's upbringing, plus stories from other mothers of rock stars. The Foo Fighters frontman explained to the crowd that his mom had never uttered a bad word in her life and that he had to teach her how. He then demonstrated for the Canadian Music Week crowd, flipping the bird. Kicking off the hour-long session, Grohl joked, "I'm interviewing my mom, by the way. This doesn't happen all the time!" What happened the first time you cursed in front of your parents?