Answer to Question 1
During an erection nerve fibers swell the arteries of the penis, allowing blood to rush into the corpora cavernosa and corpus spongiosum, while veins are compressed to prevent the blood from escaping. The erectile tissues thus will fill with blood, causing the penis to become erect. The penis returns to tits flaccid state when the arteries constrict, the pressure closing off the veins is released, and the sequestered blood is allowed to drain.
Answer to Question 2
The opinion most men have that the average penis size is greater than it really is comes from pornographic films (which tend to use the largest men they can find); from men's perspective on their own penis (which, from the top, looks smaller than from the sides); and from overestimates of actual penis size (researchers consistently find that people's estimation of the size of penises they have just seen is exaggerated; Shamloul, 2005).
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