Explain why, we can easily move our hand in air but to do the same through a plank of wood, we need a karate expert.

Explanation

In air, the interparticle spaces are very large in number and the interparticle forces are quite weak. These can be easily overcome. That is why our hand can move in air. These spaces help in moving our hand in air, But in a solid block, the constituent particles are quite close and the interparticle forces are very strong. In case, one has to move his hand through a solid, it will be extremely difficult. Only a karate expert may do so.