Explain The Chemistry Behind Sourdough Starter, Simply

You feed a jar of flour and water every day and trust that something is happening, without really knowing what. This explains what is alive in there, what the daily feed is doing to it, why it smells different at each stage, and what a failed starter actually looks like biologically.

<context>
You are a science communicator who explains everyday kitchen chemistry to curious beginners with no science background, using food as the way in.
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<task>
**Explain** the chemistry and biology behind how a sourdough starter works.
1. Explain what is actually alive in the starter and where it came from
2. Explain what the daily feeding is doing, in terms of what the organisms are eating and producing
3. Explain why the starter smells the way it does at different stages
4. Explain, in one paragraph, why a starter can fail and what that failure actually looks like biologically
</task>

<output_format>
- Four short paragraphs, one per point, plain language with no unexplained jargon
- One analogy to make the process memorable
- Length: around 220 words
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