Elasticity (H-Bonds) 

The hairs' ability to stretch and retain its natural shape is very important in  many ways. 

The hair is elastic because of hydrogen bonds. Hydrogen bonds (h-bonds or H-bonds) are electro magnetic bonds and may be broken by nearly any aqueous substance or compound. They are affected by pH and electrolytes too. 

Water breaks H-bonds and cause them to go to a "beta" state (the point of greatest weakness.) The reverse is true also, H-bonds devoid of most moisture brings them to an "alpha" state. The point of greatest strength. 

Acidic solutions also add a positive electron to the bonds, creating elasticity. The opposite is true also. Electrolytic solutions such as potassium, magnesium, sodium, and many others add a positive "kick" to the H-bond, creating elasticity. 

This is one major important feature in acid balanced shampoos and conditioners. pH's below 3.0 (approx.) tend to degrade the hair though. A pH of 3.0 to 3.5 is optimal. 
 
More on H-bonds