The smallest building-block of a living being that is capable of functioning on its own.

A cell is the smallest biological unit of specialized function capable of self-replication. A cell consists of an outer membrane (like a porous plastic bag), which encases the fluid of the cell, and the nucleus (a specialized compartment that contains the DNA and directs the production of proteins). Conversion of many cells (of either the same or different functions) at the same spot in the body can form an organ with specialized purposes, like the retina, liver or brain.