Sounds a lot like most guys I know...

Most guys you know think girls don't poop?

You must know some shitty guys...

wait...pretty girls poop!? life has changed for the worse

You know an awful lot of idiots, then.

So get a book and educate him

And Google up a copy for him of the poem "The Lady's Dressing Room", by Johnathan Swift (author of "A Modest Proposal"). It's about a dandy of his times being horrified to discover all the backdrop paraphernalia and processes a lady of the times goes through before appearing in public. The finding of the chamber pot leads to the classic lines,
"Disgusted Strephon stole away
Repeating in his amorous fits,
Oh! Celia, Celia, Celia shits!"

educate him with percussive maintenance and hope something gets through his thick skull

That's even more amusing because my mum's name is Celia ;3

then slap him with that book for disrespecting you

Oh the irony. Does he also think girls can't pee with a tampon in and that they don't fart either?

Wait, you can pee with a tampon in?

Different holes man

Do girls pee with tampons in? I feel like that would be kind of strange.

Oh my god. There are three holes, and peeing with a tampon in does not hinder anything except maybe on the occasion you get the string wet. Which does suck.

This is definitely going to be a whole lot of TMI but it's actually more difficult to shit with a tampon in because a lot of women have diarrhea during their periods and getting shit on the string is a lot worse than pee. Periods, what a beautiful wonder of nature.

You could always hold the string while you urinate/defecate. I haven't seen one that wasn't long enough to do so yet. Just remember to do so every time and you won't have it on the string.

3 holes? News to me. Thank you sex ed for teaching me about crabs instead of useful stuff :')

Hold it????? No way. I'd rather pee on the string or take it out before pooping than risk getting anything on my hands. Tampons are expensive but I'm not THAT worried about it.

My life has been a lie

Well, technically the same hole, just different tubes in said hole.

No, not different tubes in the same hole, that's a penis. There's a vaginal opening, and just outside of that is the urethral opening. Two separate pieces of anatomy.

Can U tell the hole story again?

Luna cup would solve that for most.

How would they fart without a colon? Only makes sense...

Guys have three holes; so do girls. Everyone has assholes; they're the same for both genders. Guys have two holes in the end of their dicks: One for procreation (the hole the sperm comes out of) and the urethra for pee. You can see them if you gently pull the slit at the end of your dick apart; if you have a foreskin you'll have to pull it back a bit.

Girls also have a hole for procreation (the vagina; tampons go in here to soak up the blood when her uterus lining sheds once a month), and a urethra, which is hard to see but is a hole just a little below the clitoris. Like guys, pee comes out of here.

The layout's a bit different between the genders but the plumbing's basically the same as the basic needs are the same for both sexes: Poop, pee, and reproduction.

Colons and penises.. So easily confused. He's an idiot

He could also be thinking of the prostate.

Get someone else's opinion. It's a win-win: you’ll either prove him wrong or weed out the idiots.

Why get anyone's opinion? OP could just google it, show his idiot brother, and that'd be the end of that.

Is he confusing it with prostate, or still thinks girls don't poop?

I'm guessing he came to this conclusion because supposingly he also believes girls never fart?

Well the medical field just got crossed off his brother's list of potential career paths.

Colon is a large intestine. Women have those. Women can get colon cancer.

When he gets a girlfriend, he is in for a big surprise.