Physiologically, alimentary canal (gastrointestinal tract) is lined with mucous layer. Mucous contains mucin protein and water. Mucous is secreted by mucous glands present on the epithelium. Mucous, is present not only inside the alimentary canal, but also lines the respiratory and urinogenital tracts. This mucous lining primarily acts as a protective barrier and protects the underlying living cells from pathogens and from other physical and chemical hazards.
When we drink very hot liquid this mucous lining protects the living cells beneath it from getting burnt. This happens mainly due to liquid water molecules in mucous. Water molecules have high enthalpy (E) and boiling point. This property of water molecules in mucous enables it to act as a heat absorbent and prevent the penetration of heat beneath it. Our skin is also involved in protection of underlying tissues and organs from external environmental hazards.
Skin is composed of a superficial layer (the epidermis) and a deeper layer (the dermis). Dermis consists of many cell layers and most of the layers are made up of satiated, denucleated, keratinized dead cells. These layers act as physical barriers against the invasion of pathogens and harmful substances.
But these stratified layers are not as effective as the mucous in the heat production mechanism. Because, these layers are dry in nature and contain no water molecules to absorb heat, and are permeable, when hot substances are poured on the skin, the dry horny layers manage to give little protection to the living cells from the heat.