The term 'smooth metal' can be solved by the four-letter word 'IRON'. IRON is a chemical element with the symbol Fe and atomic number 26. It is a metal that has been used by humans for thousands of years, and is one of the most abundant elements on Earth. It is often found in its natural state as an ore, and is then processed to create a variety of products. IRON is known for its strength and durability, and is often used to make tools, weapons, and other items. It is also used in the production of steel, which is a combination of IRON and other elements.