The key characteristic that distinguishes plants from animals is their ability to produce their own food through photosynthesis.

Here's why:

* Plants are autotrophs: They contain chlorophyll, a green pigment that allows them to capture sunlight and convert it into chemical energy in the form of sugars. This process is called photosynthesis.

* Animals are heterotrophs: They cannot produce their own food and rely on consuming other organisms (plants or animals) for energy.

While there are many other differences between plants and animals, the ability to photosynthesize is the most fundamental and defining characteristic of the plant kingdom.