Cells and body systems
Cells group into tissues, tissues into organs and organs into systems, so each level does a job the level below cannot.
A worked example
Cells lining an airway form tissue, the lung is the organ, and with the trachea and diaphragm it makes the respiratory system.
The mistake people actually make
A dividing population doubles each time, so the growth multiplies rather than adds. Count how many divisions fit into the time before you multiply. A tissue is cells of one kind; an organ is several tissues working together.