In order to be alive, an object (or group of objects) must meet all of the following characteristics:
- Have an organized body made of one or more cells
- Grow and develop
- Reproduce (hence the "group of objects" - not all individuals reproduce, but some must!)
- Respond to the environment around it (short, fast responses AND long-term changes)
- Take in and use energy
- Exchange gases with environment
There are other, more complex characteristics, but we'll start with these.
In addition, all living things have common needs:
- Water (which is why we spend so much time looking for water on other planets! Water = the possibility of life!)
- A habitat (NOT "shelter" or a "home" - those are more specific for animals)
- A source of energy (from sun, food, etc.)
- Raw materials (vitamins, minerals, protein, carbohydrates, etc.)
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