How did our comedies become so sad? BBC1’s new sitcom Big School (Fridays) opened with a scene that would probably tickle the ribs of many, but I, in my usual humourless way, found it depressing. Chemistry teacher Mr Church, played by David Walliams, hoped to excite his morbidly uninterested pupils about the effects of dunking a bottle of cold liquid nitrogen in warm water by using hundreds of table-tennis balls to dramatise the resultant explosion.