A few times on TV I’ve seen these people who can stick heavy metal objects (hammers, etc.) to their chests. They’re purported as having some sort of weird magnetic field about them.
Now, I just saw a show in which one of these guys was tested in a lab for any magnetic field, and tests were negative (obviously…). So, has this phenomenon ever been explained?
Glue?
magnet taped to their back.
Velcro. Easier to do a repeat performance 
Nope. They do it shirtless. And there’s nothing on their back.
(BTW, I tried doing a search but the BB kept timing out.)
Suction? Belly suction? (Rock band)
What if they swallowed one of those super-powerful rare-earth magnets? Then they could do the trick (and possibly squish their innards together, which could be a problem), and then just poop the magnet out later.
It is a combination of having moist clammy skin and leaning in the direction away from the side where the objects are applied.
I was sure Cecil had done a column on this, but I can’t find it.
Yep, I’ve deffinitely noticed the unusual backward leaning and suspected that has something to do with it. It still doesn’t seem possible. I’ll have to try it 
Also, notice how the heads of the hammer are placed on the rounded parts of the chest and abdomen, so in a sense they aren’t clinging to a 90º vertical surface, maybe more like a 50º gradient (on top of whatever they gain by leaning back ever-so-slightly). Plus like Mangetout said, the clammy skin provides the proper friction, and I also think the long handles might provide some form of balance.
Yes. The adhesion of the smooth metal hammer head on the clammy skin means that any downward motion tends to rotate the hammer in such a way as to press the handle more firmly against the skin, further improving grip.
