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like a pokemon card has value, rare vs. commons but breeding a normal cat could make a rare one. over time the older gens will be much rarer

The same way that a stamp has value, or how a trading card has value, someone is willing to buy the asset. Sometimes it is a unique feature, like a specific run, or a misprint, and other times it is utility, but as long as people are interested in purchasing the, in this case "unique" asset (each cat is basically a programmed token with it's own DNA) then it will have a value.

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