we can easily move our hand in a fluid like air. It is more difficult to move our hand in a liquid like water. It is more difficult to move our hand through a pile of sand. Then it is very difficult to move our hand through a solid block of wood. Only a karate expert can do that.
Reason is that the density of the material increases from air, to water to a pile of sand to a solid block of wood. It is more difficult to move our hand through a medium when density increases. Then the particles are close together in a medium. The particles are attracted much more towards each other, when the density is higher. To separate the particles so that the hand can penetrate through the gap between the particles in the medium, we need more force and pressure.
The karate expert, applies a lot of force on a small area of the solid block of wood. So the stress or pressure is so high that it is more than the breaking stress limit of the wood.
The solid block of wood is highly rigid, less flexible, strong, modulus of rigidity is very high. The liquids and gases have very less rigidity and are more fluid and more flexible. The particles can therefore be displaced with very little force or pressure.