Cranking the numbers gives me 1,394 nails—which is actually not that many. A 40-by-40 square grid is 1,600 nails, and that’s more than you need to prevent a skin puncture.

So, what if you replace the nails with a bunch of broken glass? It’s really the same thing. Sure, glass might be sharper than nails, but it also has some flat parts. As long as the contact area is large enough, the glass won’t hurt anyone.

So that’s the secret: It doesn’t take tough skin, just some physics.

Smashing Rocks, Mass, and Acceleration

Now, let’s move on to the part of the demonstration when a soldier smashes a rock on top of the dude’s chest as he lies on the bed of nails. The key physics lesson here involves Newton’s second law. This is a relationship between the net force on an object (F_net), the mass of the object (m), and the object’s acceleration (a). If the object is constrained to only move in one dimension (to make things easier), then we can represent this as the following equation:

The acceleration of an object tells you how that object’s velocity changes. So, if the thing is just staying still, then the velocity will constantly be zero, which would be a zero acceleration. However, even if the object is moving, it can have a zero acceleration as long as its velocity doesn’t change. If the object is increasing in speed, then it would have a positive value for the acceleration. That means that when an object slows down, it has a negative acceleration. (Note: This assumes motion in one dimension.)

Here’s an example: Suppose two people are each standing on a skateboard. (These are zero-friction skateboards—you can find them at the physics store.) On one board is an adult with a mass of 80 kilograms, and on the other is a child with a mass of 40 kilograms. If I push on the adult with a force of 80 newtons, it will provide an acceleration of 1 meter per second per second (1 m/s²). If I push with this same force on the child, the acceleration will be twice as much (2 m/s²), since the kid’s mass is half that of the adult.