Stretching, in the context of body piercing, is the deliberate expansion of a healed fistula for the purpose of wearing body piercing jewellery. Ear piercings are the most commonly stretched piercings, with nasal septum piercings, tongue piercings and lip piercings/lip plates following close behind. While all piercings can be stretched to some degree, cartilage piercings are usually more difficult to stretch and more likely to form hypertrophic scars if stretched quickly. Dermal punching is generally the preferred method for accommodating larger jewelry in cartilage piercings.
Stretching is usually done in small increments to minimize the potential for damaging the healed fistula or creating scar tissue. In North America, most stretching methods go up by a single even-sized gauge at a time. In Europe and most of the rest of the world, jewelry is metric, but the increments between standard sizes are similar.
According to Wikipedia, my original thought that you could stretch your ears up to 20mm and have them still shrink back is actually wrong....
13 millimetres (0.5 in) is normally given as the "point of no return" for earlobe piercings, as over this size there is a significant risk that the hole will never shrink back to the size of the original piercing. Many variables affect whether or not a stretched piercing will return to its original size, such as the length of time taken to stretch and the amount of time the piercing is fully healed at a particular size.
People also stretch other parts of their body, such as lips in some tribes..