Can animals laugh? Some say horses and even dogs can. However, only humans smile. Smiling begins instinctively during the first few weeks of life and is not directed at anything in particular. By a baby's fifth week, he or she smiles in reaction to certain stimuli, especially as it learns to recognise its mother.
As we grow older, we are slightly wary of any social encounter and especially ones that involve meeting someone new. Then, our natural, instinctive facial expression is a look of fear. We open our mouth slightly and the lips pull back. The simple act of curling up the corners of the mouth completely changes that expression into a smile, a sign that we are not enemies.
Friendly people smile a lot. Wary, frightened or aggressive people smile less often. Some adults don't smile because they are ashamed to show their yellow, rotting or missing teeth. One in three people has 12 or more fillings. One in four adults regularly has smelly breath. One in five people brush their teeth less than twice a day. No wonder their lips are sealed.
To control the appearance of our smile is simple: it is a matter of eating healthily. The main cause of tooth decay is not how much sugar you eat, but how often you consume it. The more often you have a sugary drink or snack, the more likely your teeth are to decay. If you must snack between meals, stick to cheese, vegetables or fruit, but not dried fruit.
Remember: 96 per cent of people see a smile as important to someone's overall appearance. Two out of three people think a healthy smile makes you more successful, and the same number are more likely to speak to someone they don't know if they smile. Smiling can make you popular.
Create posters or an actual buffet of "smiling", healthy food: slices of melon, brown bread sandwiches, pieces of cheese, shaped into smiles.
The theme of this year's National Smile Month is "feed your smile". For more information, visit the British Dental Health website www.nationalsmileweek.org
Click on "fun stuff" for print versions of puzzles and an interactive quiz.
Posters and other items are also available.