Because iron is harder than wood.
Why cannot our bodies penetrate the wood?
Because our bodies are softer than the wood.
Why can our bodies move more easily through air than through water?
Because air is more easily penetrated than water.
Why can a nail be driven into a pine board more easily than into an oak board?
Because the fibres of pine are more easily separated than the fibres of oak; hence, pine wood is more penetrable than oak wood.
