Your man-hood is simple: It only has a few functions, and it tells you everything you need to know about it in one glance.

The v**ina, on the other hand, is a mysterious thing—an elaborate, multifaceted tool that still confounds men. And while we don’t claim to have all the answers, at least we can share these five incredible facts about her private parts.

Prepare to be enlightened and amazed.

When you become hard, it’s because your corpus cavernosa—the twin tubes of erectile tissue in your man-hood—are engorged with blood.

The cli**ris also has a pair of corpus cavernosa inside of it, which means she, too, gets erect when she’s aroused. An erect cli**ris will swell, become more sensitive, and increase in size. The inside of her v**ina will also expand as blood flow increases.

“The v**ina has a series of accordion-like folds called rugae that, during arousal, allow the v**ina to increase in length to roughly the same size as the man-hood,” says Dr. Goldstein.

It’s harder to see, but her body is responding to arousal similarly to your erection, although it might take her longer than it takes you. Still, it’s worth waiting until she’s totally aroused and erect. Imagine having s*x with a half-flaccid man-hood—sound like fun? We didn’t think so.

About a third of women can recall orgasming in their sleep, according to a study in the Journal of s*x Research.

While nocturnal orgasms don’t happen all that often, nocturnal arousal occurs regularly during REM sleep—up to five times per night. And it’s not just her cli**ris that becomes engorged with blood, but the entire private part area, making her more likely to have an climax.

Though your unclad eyes won’t be able to detect them, it’s worth tapping her shoulder to see if she’s already primed for a midnight romp.