Things sink if they're denser than the water or whatever liquid they're in.
Density is the weight [or mass] of something per unit volume. Water has a density of 1 gram per milliliter.*
A marble, for example, is heavier than the amount of water that's the same size as the marble, so the marble sinks.
* The density of a substance is its mass per unit volume.
Some things that sink in water will float in salt water, because salt water, with salt + water in it, is denser than water.
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