Is rigid the same as hard?

Rigid and hard are similar but not exactly same, though often used interchangeably. Rigid means stiff, inflexible, and resistant to bending or changing shape, emphasizing lack of flexibility and ability to hold shape under pressure. Hard means solid, firm, and resistant to scratching, denting, or cutting, emphasizing resistance to deformation and penetration. Something can be hard but not rigid, for example hard rubber is hard to scratch but can be flexible, not rigid. Something can be rigid but not extremely hard, for example rigid cardboard is stiff and does not bend easily but is not hard like metal and can be punctured. In many contexts, rigid and hard overlap, as in rigid board is also hard board. But rigid focuses on inflexibility and not bending, while hard focuses on firmness and resistance to indentation. So rigid is about not bending, hard is about not easily scratched or dented. They are related but distinct properties, and context determines whether they mean same thing.