Strange Substances

John came onto the stage without his band for the first half of the gig. He sat down, took a long draw from something looking suspiciously like a joint and started strumming his guitar. One wag from the audience shouted out “Can we do that John?” obviously hoping to be given permission to smoke substances in the auditorium. John replied “If you practice for a few years, you might just manage it!” and continued to play the guitar.