Perhaps the most obvious criterion for life is being able to reproduce. If something is alive, then it must be able to produce copies of itself, which may or may not be identical. But this criterion is not sufficient on its own because crystals are able to grow and produce identical copies of themselves if placed in a salt solution. And no one would argue as to whether or not crystals are alive.
To the ability to to reproduce we need to add the ability to evolve. For something to be alive, the copies it produces of itself need to be able to change gradually across the generations in response to environmental factors. Evolution in the domesticated dog world can happen quickly, after only a few generations by selective breeding. If left to nature, the same changes would take millennia to occur through the process of natural selection.
Some questions to ask if an organism is alive: 1. Is it made up of at least one cell? 2. Does it metabolise – does it use and release energy? 3. Does it grow and change over time? 4. Does it respond to any physical, chemical or environmental stimuli? 5. Does it reproduce either sexually or asexually, and evolve? If the answer is NO to any of these questions, then what you’re looking at is actually NOT alive!
At its most basic level life must contain organic molecules of carbon and hydrogen.