Density

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Mass

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Volume

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Stuff

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Space

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Protons & Neutrons

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Size of Object

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# of grams

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# of cm³ (ml)

Water always has a density of 1 g/cm³, because
1 gram of water occupies 1 cm³ (1 ml) of space,
2 grams of water occupy 2 cm³ (2 ml) of space,
etc...