How to increase the risk of breaking it during sexual relations.

This article, newly published, explains how people are more likely to fracture the penis during stressful situations, such as those commonly found when having (and hiding) an extramarital affair.
http://onlinelibrary.wiley.com/doi/10.1111/j.1743-6109.2011.02461.x/abstract



I will refrain from posting pictures.  Per Wikipedia http://en.wikipedia.org/wiki/Penile_fracture this is most obvious from "a popping or cracking sound, significant pain, immediate flaccidity.  There will probably also be a hematoma (swollen blood filled area under the skin)  If this happens, immediate surgery is needed! 

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