When building a city you start with bricks. Many bricks joined together make walls. Walls working together make a building. Many buildings make a city. The human body is put together in a similar way. Humans start with cells. Many cells joined together make tissue. Tissue working together make an organ. Many organs make an organ system. Cells are the smallest living part of an organism. Some examples of cells in the human body are brain cells, lung cells and bone cells. When cells