They're simply groups of cells held together by electrical charges that keep the cells in place so that a piece of this page or a piece of your finger doesn't go spinning off into space. Individual cells stay intact because they have a cell membrane, an outer skin that serves as neat and tidy packaging for the cell. One requirement for healthy cell membranes is ? drumroll please ? cholesterol. A whopping 90 percent of all the cholesterol in your body is in your cell membranes. The cholesterol protects the integrity of the cell membrane, helping to keep it flexible and strong. If you were to diet so stringently or use so many cholesterol-lowering drugs that your cholesterol level fell to zero (an impossibility by the way), your cell membranes would be very dry and easily torn. The stuff inside the cells would leak out, and cells would die all over the place. That would sort of put an end to the whole darn shootin? match. Every healthy body cell needs some cholesterol, and so does every healthy brain. Atoms are the basic building blocks of elements ? hydrogen, oxygen, carbon, and all their chemical cousins. Each atom carries the name of the element it represents (such as hydrogen). In addition, each atom has a shorthand symbol ? call it a nickname ? such as H for hydrogen. Sometimes, an atom's shorthand name seems totally divorced from its full name. For example, lead atoms are called, well, lead atoms, but the symbol for a lead atom is Pb, from plumbum, the Latin word for lead. There are also elements and atoms named for human beings. For example, seaborgium is named for Nobel Laureate Glenn T. Seaborg; its shorthand symbol is Sg. Individual atoms form bonds with other atoms to create clusters of atoms called molecules. To write the name of a molecule ? its formula ? you write the symbols of the different atoms that the molecule contains and the number of each type of atom right after the symbol. For example, if I write H2O, the formula for the water molecule, you know immediately that a water molecule has two hydrogen atoms and one oxygen atom. A body cell, the smallest independent unit of a living creature, is a collection of molecules. And you, wonderful reader, are a collection of cells.