A few people have e-mailed me about this.
What you're looking at, here, is a sandwich. The sandwich's components are:
1: One large neodymium-iron-boron ("NIB") magnet.
2: The fingernail and finger-tip, mashed down to negligible thickness, of a gentleman called Dirk.
3: Another large NIB magnet.
Hackneyed though it is to say this, Dirk was lucky. He could have been luckier, I grant you, but he could also easily have lost a whole hand to rare-earth magnets this large.
You can read the whole grisly story here on the MagnetNerd.com site, the front page of which shows a couple of little half-inch-cube NIBs attracting each other through the thickness of a man's hand.
(The Magnet Nerd also has good pages about magnetic perpetual-motion machines and the various other evergreen magnet scams. And a line of chunky wooden implements to let you handle large NIBs, and pull the blighters apart, without losing any digits.)
The mutual attraction between magnets of all types increases, all other things being equal, exponentially as the magnets get bigger, and approximately with the inverse-cube of the distance between them. (That last part is the bit that really sneaks up on you.) So you can make an instant earring by sticking a couple of small NIB discs together with your ear-lobe in between, or you can smash your whole hand into wafer-thin steak tartare with a couple of magnets the size of cigarette packets.
I think it's brilliant that anybody can buy fist-sized NIB magnets from a variety of dealers - Forcefield ("WonderMagnet"), Engineered Concepts ("SuperMagnetMan"), and of course umpteen eBay dealers. I think the people selling them are generally very responsible, too; in product listings for the big buggers, there's usually a BE CAREFUL YOU IDIOT warning. And to my knowledge NIB magnets are generally very well packaged, too - collections of small magnets get a mild-steel wrapper, and really big magnets get great big double-boxed packaging, firmly holding the magnet in the middle of a large box.
The biggest magnets in this house are the two-inch-square trapezoid and two-by-one-inch cylinders from this old review. I don't know whether you could actually smash all of the bones in your hand by putting one of the cylinders on one side and one on the other. You'd probably just get a very nasty bruise. I'll leave the experiments involving gauntlets, eye protection and supermarket poultry to someone else, though. And the two-inchers ain't nothin' compared to what's on offer these days.
As I write this, a quick eBay search (if you use the not-often-useful "Price + Postage: highest first" sorting option) turns up a 4-by-1.5-inch disc for $US169.99 ex shipping, 2-by-2-inch cylinders for $US109.99, and, most terrifyingly, a two-inch sphere for $US139.99.
Spherical magnets have the problem that only a tiny area of their surface can be in contact with any other object - like another magnet - that isn't concave. For little sphere magnets - the quarter-inchers, for instance, that you can use to make impromptu rings or bracelets - this just means that they need extra-thick nickel plating so the little contact patches between the spheres won't quickly wear down to the brittle black ceramic of the magnet material itself.
A big NIB sphere, though, is aching to smash itself into other magnets just like every other big NIB, but is fated to deliver all of its terrifying impact energy to that one tiny contact point.
I imagine the X-rays of that victim would look quite interesting.
If you reckon it's time for everyone to start calling you "Lefty" or "Stumps", NIB-magnet dealers stand ready to assist you. The Engineered Concepts guy currently has a 6-by-1-inch ring magnet for $US425, 6-by-4-by-0.75-inch blocks for $US325, and wedges for the bold wind-generator maker (find info about this at Forcefield's other site, Otherpower.com) at $US720 for half of an eight-inch-outside-diameter ring.
Forcefield, meanwhile, will be pleased to sell you wedges suitable for making a 14-inch ring, for $US30 each. The rest of their range tops out around the two-inch size class.
(If you're for some reason not seized by an uncontrollable urge to maim yourself in an unusual way, I suggest Forcefield's $20 Grab Bag. It contains an assortment of different NIBs, none of which are big enough to give you anything worse than a blood-blister. If you're buying for a child - preferably one who's old enough to avoid swallowing more than one magnet - I suggest getting a large number of quarter-inch-or-smaller discs or cubes. They're cheap these days, and a lot of fun.)
