Three USA educational institutions were commissioned by the
government to discover why the human penis is the shaped the way it is.
Colgate University allocated a budget of $500,000 for research. After 2
years they concluded that the reason the head of the penis is wider
than the shaft is that it fits better, when in situ, so to speak. This would
prevent leakage of semen and increase the probability of successful
fertilization.

The University of Texas spent $750,000 on a research program that
lasted 3 years. The results showed that the penis widened near the tip because
it maximized the number of nerve endings stimulated during sex. This
would lead to increased sensitivity and a better chance of impregnation.

Finally, the Texas A&M University spent $3.95 on a copy of Playboy and 10
minutes in the staff toilet, only to discover the penis widens at the
tip in order to prevent your hand from slipping off the end