The characteristic that classifies *Escherichia coli* as a living thing, despite its small size, is its ability to carry out all the essential functions of life. These include:

* Metabolism: *E. coli* can take in nutrients, break them down for energy, and build new molecules needed for growth and function.

* Growth and Development: *E. coli* can reproduce and increase in number.

* Homeostasis: *E. coli* can maintain a stable internal environment despite external changes.

* Response to stimuli: *E. coli* can sense and respond to changes in its environment.

* Adaptation: *E. coli* can evolve over time to better suit its environment.

While *E. coli* is a single-celled organism, it performs all these functions necessary for life, just like larger, more complex organisms.

Size is not a defining characteristic of life. There are many organisms, like viruses, that are smaller than *E. coli* but are not considered living because they lack some of the key characteristics listed above.