Flexibility...

Ok, so we know by now that it is important to have a certain amount of flexibility.  Stretching, obviously, helps us attain this flexibility.

Stretching is a type of mechanical loading, and the effects of this loading on the collagenous tissue reveal the science of stretching. (For lack of a bigger forum, collagen is basically connective tissue - its fibers provide strength and stiffness).

Physiological stretching is possible because collagen is a viscoelastic material.  Under rapid loading it behaves elastically, while under slow, or gradual, loading it is viscous and can deform plastically.  A quick and intense displacement of the joint can result in a muscle tear and/or joint dislocation.  However, a safe and gradual elongation to a point of mild discomfort will eventually lead to the desired deformation and produce a lasting (not permanent) flexibility.

By the way, it is believed that too much flexibility can lead to chronic joint pain while too little flexibility leads to recurring muscular strains and injuries.

Lastly, Just because I think this is one of the most hilarious moments in movie history: