Here's why:

* Cells are the fundamental building blocks of life. All living organisms, from the smallest bacteria to the largest animals, are made up of cells.

* Cells carry out the processes necessary for life. They have their own DNA, can reproduce, and perform essential functions like respiration and metabolism.

* Non-living things lack these characteristics. Rocks, water, air, and other inanimate objects do not have cells or exhibit the properties of life.

Therefore, the presence of cells is a defining characteristic of living organisms, and non-living things do not possess this characteristic.