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minutes: 15
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# Photosynthesis

## Warm-up: what plants eat

Ask the class where a tree's mass comes from. Most say the soil. It is air.

## ! The two stages
> key: Calvin cycle

Light reactions capture energy. The Calvin cycle builds the sugar.

## ! Why it matters
> key: oxygen, the air you breathe

Nearly all the oxygen you breathe came from this reaction.

## Wrap-up

Exit ticket: one sentence on where a tree's mass comes from.