Chilli fresh, dried, harissa paste or a slurp of bottled chilli sauce adds oomph to a noodle broth or salsa, Moroccan cous-cous and turns Thai curries into fiery hot-pots.

Pepper, ginger, mustard and horseradish (think Japanese wasabi) do the same thing, but chilli is the strongest and most widely consumed of these heat-additives.

Chillies' fire is derived from capsaicin, a group of five related capsaicinoids, which have the ability to make our eyes water, our noses run, our mouths burn and any exposed skin sting for hours.

Capsaicin stimulates the trigeminal nerve, the main facial nerve system which has branches in the mouth, nose and eyes. This nerve system is a separate sense, distinct from the senses of taste and smell, and it produces a variety of sensations which we describe as hot or pungent.

Researchers believe that this sense is one of our oldest, designed to warn and protect us from potentially harmful substances. It also initiates defence mechanisms to get rid of the ``oral irritant'', like the tears and runny nose that often accompany a hot curry.

Why we actually like even crave something that causes discomfort still puzzles scientists. One theory is that the more you eat chilli, the more you enjoy it. What was once an unpleasant burning sensation gradually becomes a pleasurable tongue-tingling way to eat.

In chilli-eating nations like Mexico, young children are introduced to chilli from an early age and preference for it develops as they grow. A plate of chopped chillies is put on the table and each child adds as many as they want, increasing it to adult level over the years.

Catherine Saxelby is a nutritionist and author of Nutrition for Life. Get more healthy eating tips at www.foodwatch.com.au