explain why, a person can lie on a bed of nails if there is a very large number of nails? 

When a force acts over a large area of an object, it produces a small pressure. But if the same force acts over a large area of an object, it produces a large pressure. The force produced by the man's weight is distributed over hundreds of nails of the nail bed. Therefore, the force applied by individual nail is reduced. As a result the force per unit area at the tip of each nail is less than that required for the nail to pierce into the skin. This implies to all the nails in contact with the man’s body. The nails do not penetrate in the man’s skin and he does not get hurt. Thus, a person can lie on a bed of nails if there are very large number of nails.