Because of the difference in density throughout space one substance is enabled to penetrate another substance and that is movement. Space itself is immovable. If all matter in space were equal in density there could be no penetration and consequently no movement. A piece of steel can penetrate water or air because it is of a different density but cannot penetrate another piece of steel of equal density. When density is equal there can be no penetration.
Steel can penetrate heat and heat can penetrate steel because they are of different density.
Man, composed of different proportions and variety of substances than air or water, is of a different density than either of them. Air being of lesser density than water, man penetrates air with less effort than he penetrates water. Man cannot penetrate steel because steel is of a greater density than man, and its crevices are not large enough to permit the body of man to pass through. Heat, which is of lesser density than steel, penetrates it in the same manner that water penetrates a metal sieve. The difference in density of heat and water is so great that heat is able to pass through the minute crevices of solid steel whereas water is unable to do so.