We had a BBC crew come to film our science class. Just before they arrived our science master said he would put questions, if we knew the answer, left hand up, if we didn't know, right hand up, so the viewers would think all his class knew all the answers. I wonder if the producer ever found out ...My father's old PA had an actress friend whose job was to sit in the audience in TV variety and magic shows and put her hand up if no one responded to "Can I have someone from the audience please?"