A video is doing the rounds on Reddit that shows a man with the apparent ability to put his hand in molten metal without being injured.
In the footage, he moves up to a stream of liquid metal, removes his safety glove, and proceeds to shove his hand into the red-hot liquid. After, he walks away like it's no big deal, completely uninjured.
Don't try this at home, even if your home does happen to contain an industrial molten metal sluice.
So how is he able to do this without melting his arm or giving himself extreme burns? Is he some kind of reverse-terminator that's resistant to molten metal? Sadly, no. However, the answer is still pretty cool.
The ability is most likely due to the Leidenfrost effect. The Leidenfrost effect is what causes water droplets to dance around on an extremely hot hob, rather than instantly evaporating.
When a liquid is dropped onto a hot plate, the bottom layer of liquid instantly evaporates. This leaves a protective layer of gas between the hot plate and the rest of the water droplet, preventing contact between the two and slowing heat transfer.
It's explained rather neatly in this video from British TV show QI, hosted by Stephen Fry.
The same effect protects the man in the video, who swipes his hand through molten metal like it's an ordinary tap.
For an (extremely) short amount of time, the moisture on his skin boils instantly, forming a protective layer on his skin and stopping him from touching the liquid metal directly. This allows him to swipe his hand through it without being burned. If he did so for even a fraction of a second longer, however, it is likely he would get burned badly. So don't try this at home.
The same effect can be seen with liquid nitrogen. As liquid nitrogen boils at -196°C (-321°F), it instantly vaporizes when it meets a warm surface (e.g. a floor or a human hand). This creates a layer of gas between itself and the surface. As a result, there are a lot of cool videos on YouTube of people pouring liquid nitrogen onto their hands, and, horribly, a few videos where people put their hands into nitrogen for too long and it all went badly wrong.
Here's one of the cooler videos, where the Leidenfrost effect worked perfectly.