This helpful and informative video will teach you just what exactly causes an erection. With a little assistance, of course.
From the Internet show "Midwest Teen Sex Show" (Relax. That's just the title. It's a comedic sex ed show geared toward a teen demographic, and there are no actual teenagers involved in the production of the show in any way, shape or form). Actress is Britney Barber, a 25-year-old comedienne. Also attached is a version with just the good stuff.
Enjoy
Fango