A mould is a microbe. It is a fungus that grows as tiny threads called filaments.

A mould grows when a spore (the 'seed of a mould) floats through the air and lands on a source of food. Normally the food we eat is not left about long enough for moulds to grow. Most are harmless anyway. But if you leave an apple or another kind of fruit out for too long, or you leave a loaf of bread in a plastic bag for several days in the warm, then you will provide the ideal conditions for a mould to grow. You will have given it a source of moist food and warmth. What happens next is that a patch of discoloured (say) bread appears, which grows over the days and changes to become fuzzy. The fuzzyness is the result of the filaments growing longer and longer. You can see that in the picture above.

The filaments are usually transparent, which is why they appear fuzzy. It is at this stage that they produce 'seeds' called spores which float off into the air. They are too small to see.

Moulds are a perfectly normal part of the life cycle. They are the part which breaks down dead matter and releases nutrients so that it can be used by other things. You cannot normally see it, but these moulds are vital to life in a soil. It is the fungi that break down the leaves in the soil, and the filaments often connect to plant roots, feeding them directly with nourishment. The giant trees in a rainforest, for example, all depend on tiny moulds growing in the soil.

Moulds (for example yeast) can be used to make bread. They can also be used to make medicines. But some moulds also cause unwanted problems in people, such as discoloured toenails. But if you eat a large amount of mould, it can be harmful, as well as making the food taste unpleasant. That is one reason people down the ages have tried to preserve food using vinegar, salt, sugar, smoking and the like. Moulds cannot grow in acid conditions produced by vinegar. Salt and sugar pull water out of food and so on.