“From snow testicles”
You'll have heard the phrase "A snowball's chance in Hell" and, since you've had to come here to find out what a snowball is, been baffled by it. Well, we're here to help. If you've ever been to Hell, you'll have noticed a distinct lack of snowballs there. Well, actually you won't, not knowing what they are, but everybody else will have been saying behind your back "Have you noticed the distinct lack of snowballs here? Yes, me too. In fact everyone has except that guy over there. What's with him?" This undoubtedly led you to conclude that Hell is not a very nice place and that the people there are unfriendly, always whispering, and probably up to no good. Perhaps you even decided never to darken Hell's doorstep again. Which would be a pity, because you'll have heard another phrase: "When Hell freezes over..." Well, when Hell does freeze over there will be snowballs there, and probably, folks there being folks, snowball fights. It'll be a lot of fun.
Astronomers have called comets "Great big dirty snowballs". Perhaps you've heard the phrase "You'll have someone's eye out with that"? Well, if an astronomer used a comet in a snowball fight, they'd almost certainly have someone's eye out. In fact they'd have someone's everything out and a lot of other people's too. Things can get very messy when you do this and really, it's best left to professional astronomers who, luckily for the rest of us, don't get asked to snowball fights very often.
So there it is: a comet isn't like a great big dirty snowball, otherwise Astrologers, who are kind of first cousins to astronomers, would be following snowballs through the sky loaded down with gold and frankincense, and that other one that nobody remembers. We wouldn't be able to move for Astrologers at Christmas and all the hotels would be booked out.
Fortunately this rarely happens, and this is, in the main, because snowballs are something else entirely. Thanks for asking. Also, for some awkward reason *shifty eyes* Snowball is the name of a kid who will rule the world some day. BOW TO ME!! Eh.. I mean.. him... he he.