We can determine if something is a living thing by seeing if they show some of the living characteristics. These characteristics include:
Reproduction - to make more of the same organism
Sensitivity - the ability to detect and react to stimuli
Nutrition - eat and digest food.
Movement - the ability to change its position or location
Respire - to breathe / gas exchange
Excretion - get rid of toxic / materials in excess
All living things show the characteristics above.
For example, we can be sure that robots are not living things because they cannot excrete, respire, or reproduce even if they can move and sense stimuli from outside. We can also be sure that plants are living because they show all characteristics, even for movement, we don't obviously see that, but for example, they may grow towards a light source.
