Hair's curliness or straightness is dictated by the hair's sulphur bonds, which can be reshaped with chemical processing, and hydrogen bonds, which can be reshaped with heat and products. Allow moisture into the hair – from the tap or from dampness in the atmosphere – and you undo the changes made to the hydrogen bonds by heated appliances and styling products so hair takes on the form dictated by the sulphur bonds.
Once moisture's inside not only do you lose the style you've created, but the water makes the hair expand, resulting in a thick, coarse texture. So the key in avoiding frizz is to protect the cuticle that keeps normal moisture in and external moisture out.
* Pulling, harsh drying, over-styling and chemical applications all disrupt the cuticle, so keep them to a minimum
* Use specialist smoothing and defrizzing shampoos and conditioners – they contain ingredients to coat hair and help prevent moisture getting in from the outside
* While hair's still wet, use serums to keep moisture in as well as building up those defences against external moisture. Avoid fine-tooth combs, though, as these can stretch and break hair, opening up a splintered gateway to moisture
* Leave hair to dry naturally or, if you're straightening, dry hair downwards so the cuticle is kept flat. Ionic dryers are helpful in smoothing hair and neutralising static