By CasperTG [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
Urethral stretching is a practice some people enjoy, but others would find too painful to even think about. The idea is to stick larger and larger things into your peehole to stretch it. Some women have stretched their urethras to the point where a penis can be inserted. Some men have surgically expanded their peeholes to accommodate items larger than they could before. For men, there are two restrictions on urethral stretching. The first is the opening itself, the meatus. The second restriction is about 1/2-inch (1 cm) into the urethra. Once past these two obstacles, items can slide in several inches without interference. Just like sounding, this needs to be done with caution to avoid both physical injury and infection.
See also a memoir of urethral stretching: Peehole Stretcher
