How do you sell when nobody's buying? That was the problem facing a bunch of American florists in the middle of the Great Depression. And then some marketing wizard came up with a new slogan, and "Say it with flowers" became part of our language. But, like most advertising claims, the promise that the act of handing over a bouquet would solve our communications problems was a false one.
Consider this scene from any number of post-war movies: Joe has been two-timing his fiancee, and she is beginning to suspect. So he arrives on her doorstep, flowers in hand, and tells her she is the only girl for him. Two minutes later, he is walking down the street picking pieces of petal from his face. What went wrong?
He spoke his lines perfectly, and the flowers said their bit, just like the florists had promised. But what were his hands saying?
Joe's hands, feet, eyes and neck - every visible part of him - told a different story. The audience picked up the signals, and so did his fiancee.
Joe's problem is similar to the one teachers face every time they subject themselves to the scrutiny of 30 pairs of eyes. They may be in control of what they say (subject, of course, to ever-tighter curricular requirements), but are relatively powerless to influence what customs officers refer to as "non-verbal leakage" - the unconscious signals our bodies emit while we are worrying about the words we use.
One way to confront the problem might be to compare the body language of effective teachers and their less effective or experienced counterparts.
Sean Neill began his postgraduate career studying animal behaviour, but soon became interested in identifying comparable patterns in young children. It was a short step to observing the people who teach them, and today he is an acclaimed expert on classroom interactions.
In the early 1990s, Mr Neill videoed teachers at all levels of experience, studied their behaviour in the classroom and, with Chris Caswell, documented his observations in a handbook called Body Language For Competent Teachers (Routledge 1993, out of print since 1996). In it, he describes in detail how effectiv teachers can use non-verbal signals to secure a position of dominance in a classroom ("dominant does not mean domineering", he points out).
For example, he notes that effective teachers tend to be more decisive and more relaxed during confrontations. They are less likely to shout, and they use non-verbal techniques to defuse situations. Their manner is confident and relaxed. Often they stand in a casual pose, leaning against a desk or with their hands in their pockets.
They don't pace up and down like caged beasts, fumble or rub their faces, indulge in anxious self-grooming or hide behind piles of books - all unconscious actions correctly interpreted by pupils as signs of weakness, uncertainty or downright terror.
Effective teachers "convey by their manner that they expect the attention of the class", says Mr Neill. "Communicating relaxed control demands considerable subtlety in posture and timing."
Of course, knowing how confident teachers behave is one thing, but acquiring that confidence is another. And there's a risk that becoming painfully aware of one's awkward gait and anxious mannerisms will have the opposite effect.
"Dominant individuals appear relaxed," Mr Neill says, "and beginning teachers are often urged to behave casually - just to do what they would do normally if they were not standing in front of a class.
"The problem is that very few of us are aware of what we do normally. As soon as we start thinking about what we are doing, our behaviour is liable to change." One answer, perhaps, is to take a lesson from method actors: to persuade yourself you are that relaxed, confident and casual. Indeed, film producer Lord Puttnam, chairman of the General Teaching Council, has proposed that out-of-work actors might have a useful role to play in helping teachers improve their presentation skills.
Mr Neill says: "The way to give a convincing performance is very often to get yourself into a convincing frame of mind. If unconsciously you've convinced yourself, then you don't have any anxiety to betray. To be convincing, you need to be unaware that you are being deceptive. And to have a controlling role, you have to behave like somebody who has a controlling role."