We size them up a lot, but for all the attention guys get for what they're packing below the belt, it turns out most of us still know pretty much nada about the penis.

So in the spirit of ever-growing our knowledge about sexual organs, we've rounded up seven seriously weird facts about penises.

(And believe us, you'll never look at one the same way again ...)

1. Penises used to have hard spikes.

But guys lost them before we — as homo sapiens — evolved from Neanderthals. Scientists speculate that the penile spine helped quicken the time it took for a man to get an erection since it's commonly found in promiscuous species like mice and monkeys.

Researchers from Stanford University in California found that we lost this molecular mechanism from our primate ancestors eons ago.

It's all about survival, right?

2. The "seam" on a penis used to be a labia.

Remember how we all start out as females in utero? One of the remnants from a man's few weeks as a fetal female is the "seam" on the underside of his penis.

In women, that seam becomes the inner lips of the vagina.

But in men, that seam encloses the urethra along the length of the penis.

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3. Men can be born with two penises.

Diphallus, a rare and unfortunate medical condition that affects approximately one in five to six million men, is when a man is born with two penises.

Some guys might think of this as winning the lottery, but hold your horses, fellas. It's rare that both penises are fully functional.

So ... whomp whomp.

4. There's a species of fish that swims up penises.

When we refer to his "swimmers," this isn't exactly what we mean.

Candiru fish, native to the Amazon, have a legendary habit of attacking men who pee in the river by swimming up their urethra. In one case in 1997, a man reportedly had to undergo a two-hour urological procedure to have one removed, but it is still hotly debated as to whether or not these reports are true.

Anyway, have fun sleeping tonight, guys.

5. Napoleon Bonaparte's penis was auctioned off.

We bet you missed this part of history class.

When the French Emperor died in 1821, his doctor cut off his penis for the autopsy. Then, in 1977, the penis was sold at auction for a whopping $3,000 to a urologist in New Jersey, where he kept it for 30 years (just, why?).

Needless to say, a 156-year-old penis doesn't hold up well over time.

It was even put for up display back in 1927, and a TIME writer said it looked like a "maltreated strip of buckskin shoelace." People have also compared it to leather, a shriveled eel, and beef jerky (thanks for forever ruining beef jerky for us).

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6. The fear of "shrinkage" is a real thing.

A lot of guys worry about size, and that's pretty normal. But this is a whole new level of anxiety.

Koro is an odd, delusional syndrome in which a man is overwhelmed by the debilitating fear that his penis will shrink down to the point of disappearing altogether.

7. The English word for "penis" is still pretty new.

We've used plenty of euphemisms over time, but the clinical word "penis" (meaning "tail" in Latin) wasn't adopted into the English language until the 17th century.

Prior to that, a guy's package was referred to as a "yard" or "pizzle."