I remember seeing a bed of nails in high school Physics. At first, it seems baffling that a person could lie on a bed of pointy nails and not be injured...but it's a matter of distributing your weight across many nails so they don't pierce you. The Exploratorium's Zeke Kossover explains in this video: "It's [safe] because you're not lying on a bed of one nail."
This video climaxes around 6:00 with a second bed of nails sandwiching Kossover, and Norm from Tested smashing a concrete block on top of that using a sledgehammer (!). Enjoy:
Fun bits in this video: Kossover determined the density of nails in his bed by using a banana due to its similarity to human skin (eek!), "longer nails look more impressive," and a peg-board is a smart way to make the bed, so you don't have to hammer a zillion nails.
For more Physics details, check out this Popular Science article.