Why does applying heat to hair allow it to curl (such as with a curling iron or hot curlers)? For that matter, what makes an iron get wrinkles out out of fabric? I have always wondered…
Dizzy
The History of Perms http://freespace.virgin.net/david.astling/AHistoryofPermanentWaving.html
http://www.health-library.com/hairloss/part8.htm
From the second site…
“Permanent waving rearranges the inner hair molecules, breaking and reforming its sulfur bonds, in a step-wise chemical process (that gives off the familiar sulfide odor which wafts off the head being waved).”
And if you want to get really technical about the makeup of your hair and its’ it constituant parts…
http://www.salonweb.com/pro/damage.htm
Yes I do realise that I never answered your question…
Hair curls due to its shape in cross-section. Round hair is straight, and ovoid hair is curly. And there are variations in between.
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I knew why hair curls due to perms and genetics… but I still need to know why straight hair curls when you use a curling iron (and then goes straight again when you get it wet). So I am still wondering 
Dizzy