The Leidenfrost effect
is a cool phenomenon
that will allow you to comfortably dip your hand into molten lead
(400+ degrees Celsius
- 700+ degrees Fahrenheit
This stunt involves no trickery. All you need to do is wet your hand first. As soon as the performer's flesh touches the hot liquid metal, the water on his/her hand is vapourized, coating the fingers with a vapour layer. If the dip was only brief (a few seconds), then the flesh can not be heated significantly because the vapour layer is not very conductive of heat.
This is called film boiling. It can also be used to pour liquid nitrogen into one's mouth without being hurt by its extreme cold. From the heat of your mouth, the liquid immediately undergoes film boiling on its bottom surface and does not directly touch the tounge. This stunt is especially cool because if you breathe out of your mouth, the moisture in your breath condenses and you create a terrific, 5-foot-long plume - like on a very cold morning.
This Leidenfrost effect may also play a role in firewalking. It's just physics, folks.
Editor's note. Do not swallow liquid nitrogen. Do not try any of these tricks at home.