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A glass of water

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You have a cup of red sand, and you pour it into a bucket of blue sand, and mix thoroughly. You then take a cup of the mixture.

The chance that you get at least one grain of red sand in your cup is clearly close to 100%.

Similarly, if you drink a glass of water, the chance that the water contains at least one molecule of water drunk by Cleopatra on a particular occasion is also close to 100%.

The calculation