Because everybody knows that unicorns like cheddar, better? 
FWIW, I looked up coefficients of linear expansion for brass and iron, and did some back-of-the-envelope calculations. If we assume a large (although obviously fictional) brass “monkey” and a temperature drop of 150 degrees F, the relative shrinkage between it and the (also fictional) pyramid of iron shot is around half the thickness of a dime.
So, it would have to be really cold?
:rolleyes:
That does make sense, so you may be right, but then surely they’d have just pointed that out, instead of … well, all that argument up there…
Brass Monkey - that funky Monkey
Brass Monkey - junkie
That funky Monkey
It depends on why you are trapping them. If you want it alive you use Swiss cheese, if you just want the unicorn for the sparkly rainbow meat you use peanut butter.
Colder than a witch’s tit.
Witch’s tits are actually pretty hot. Which is why they have to keep 'em in a brass bra.
Cold enough to freeze the balls off a pool table.
How do you know if you’ve captured the Invisible Pink Unicorn?
By listening for the inaudible hoofbeats, obviously.
And a well-digger’s ass.
No.
No.
No?
No, ***WHAT?! ***
No isn’t an answer, did you not read the entire thread?
Not our finest moment, Dopers. Not our finest moment. Fighting ignorance, indeed.
My ignorance was fought quite nicely. I did not know anyone believed such a ridiculous story about brass monkeys, and now I do. I didn’t know Mangetout could be so picky, and now I do. I didn’t know the difference between iron and brass in cold weather was so tiny, and now I do.
Thanks, everyone!
Meh. Definitely not our worst, though! 
In one of the first threadsI ever participated in here, we got into a discussion of what’s coldest: the balls on a brass monkey, a witch’s tit, a welldigger’s ass, hell’s bells, or a husky’s nuts in a dogsled race.
No votes for a penguin’s chuff?