Mrs Gren is a fictional woman who serves as an acronym for the seven properties a thing needs to have to technically be alive

Movement -- the ability to move through space
Respiraton -- the ability to obtain energy through biochemical reactions
Sensitivity -- the ability to respond to stimuli

Growth -- the ability to increase in size
Reproduction -- the ability to create more living things
Excretion -- The ability to dispose of waste chemicals
Nutrition -- the ability to extract useful chemicals from the environment

This is the simple view, the biologist's view of life. It is not uncontroversial. Notice, for example, that viruses aren't a life form according to this definition -- they don't grow and don't respire. Bacteria are considered alive, though.

This definition might not hold as we move towards certain artificial intelligences that fulfill very few of the recognised catagories above. It seems perverse that an Aibo is less 'alive' than a bacteria.

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