A friend showed me the following video a few minutes ago. He says that it's real, but I can't believe that actually. What do you think?
The video show some guy hammering a nail into his nose.
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Howstuffworks has a pretty good explanation of this:
The external portion of the nose is a little deceptive. Two nostrils appear to lead almost straight up into the nasal passageway. In reality, the nasal cavity, which connects the nose to the throat, leads almost straight back. Its ceiling is approximately even with the top of the nose, just below the eyes. Its floor tends to be almost level with the alar cartilage, which forms the openings for the nostrils.
As you can see in this image, there's plenty of space, but there might be some need to "wiggle" the spike in and then push up the nostrils a bit.
It is quite likely true. It is a common circus/freak-show stunt.
The skill is called the Human Blockhead. ("Skill", not "trick", because the nail is real and does go into the head.)
Here's a video of another guy inserting a 4-inch nail into his nose in a similar way.
It involves inserting the nail into the nasal cavity, and relies of people not understanding the anatomy of the skull for its impressiveness. This How Stuff Works article goes in detail about where to stick the nail.
Scam School also demonstrated how to do this a while back.