Q. you can lie on a bed of nails without hurting yourself. Why could you not walk on the same bed of nails?

Interesting question and not anything you should ever try at home. 

Basically, it has to do with the even distribution of a person's weight over a small, sharp area - in the case of a singular nail, or over a broad area, as in the case of a bed of nails.  By distributing one's weight over many, many sharp points, one may avoid injury as the pressure threshold required for the nail to pierce the skin is not met at each individual point.  On the other hand, when you step on a nail, your entire body weight rests on the tip of one nail at one singular point.  That is cause, for most of us, to be injured by the nail.

Here's an entry we found via literati, which you may look up yourself here, if you have a valid library card number.

http://literati.credoreference.com/content/entry/chambun/bed_of_nails/0?result=0

I've included the citation information, in case you need this for a paper.

"A board studded with nails on which fakirs lie.

A bed of nails is a rectangular wooden board with a large number of nails mounted on it so that their points protrude upwards. It is so called because a person lies down at full length on it, seeming to avoid being injured by some inexplicable means.

This is a feat originally associated with Indian ascetics, known as fakirs, undertaken to show the dominance of mind over body. Conjurers also performed it and it became part of the repertoire of ‘magicians’ all over the world, who would often elaborate on it by applying weights, or another board of nails, on top of the body.

The reason that the nails do not puncture the person’s skin is that their weight is evenly distributed over the nails. The same principle applies when, as in magic stunts, another nailed board is placed on top of the person and struck with a heavy blow. Most of the force is absorbed by the board itself, with the rest being evenly distributed over all of the nails that are actually in contact with the person’s body. If, for example, the person put all of their weight on a hand or any other small area of their body they would certainly risk injury."