So sang Dick Van Dyke, playing Bert the chimney sweep in the classic musical fairytale Mary Poppins.

Few of the millions of excited children - and their mothers and fathers who watched that enchanting film - would dispute his words.

Even today, when sweeps are a dying breed, most Westerners are familiar with the idea that somehow chimney sweeps manage to spread good fortune wherever they travel.

A sweep remains a popular figure at weddings (more of which later) and indeed some reckon to earn just as much from brushing past the church door on cue as they do brushing soot. But how did this intriguing snippet of folklore start? And how has it managed to persist into an age in which fewer and fewer houses depend on wood and coal?

Like many superstitions it stems from ancient beliefs in the sacred nature of ordinary everyday objects or features of nature, in this case the household fire and hearth. Fire has always been a phenomenon to respect - even worship - and for thousands of years it has been a symbol of life and survival across the world.

Tradition has it that a sweep should be greeted warmly or bowed to. Some say that you should also spit and make a wish. But there are drawbacks. It is essential that you first catch sight of him walking towards you. If he's walking away it is a sign of bad news to follow.

To spread his luck he must also be in full work gear, sporting a blackened and grimy face. A clean sweep, pressed and dressed, is no use to the superstitious because his powers are said to fall off with the clothes he uses on the job.

Superstitions, by their very nature, cover an enormous range of everyday items and events. They may be irrational, but it doesn't follow that they are valueless. They may be taken too seriously, yet it is rare for them to do any serious psychological harm. The comforting thing about them is that for every action that brings bad luck, the chances are there is a 'charm', or antidote, to send it away again.