Why are there three generations of fermions?

All ordinary matter, include every atom in your body, is composed of just 3 different fermions: electrons and two types of quark, called up and down. The strange thing is that this triplet is just the first of 3 generations of particles, each heavier than the one before. The second- and third-generation particles are very short-lived and quickly transform into their lighter, stable counterparts. But apart from their mass and lifetimes, there’s really no difference between the three generations. It’s as if the laws of nature were written in triplicate. Why is that?

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