The term 'small building block' can be summarized by two crossword solutions: ATOM and LEGO. An ATOM is the smallest particle of an element that still retains its chemical properties. Atoms are made up of protons, neutrons, and electrons. LEGO is a type of plastic construction toy consisting of interlocking pieces that can be used to build models of various shapes and sizes. LEGO pieces are often referred to as 'building blocks' due to their versatility and ability to be used to construct a variety of structures.