The famous Irish lore is based on a bit of trickery -- there really is no end to a rainbow. The way the physics work, rainbows are actually full circles -- only the Earth's ground gets in the way of us seeing the complete circle.
One version of the fable is that a leprechaun granted one wish to a couple who was very poor and had little. They asked for riches of gold and silver. The leprechaun was angered that the couple would be so selfish, but as a hint, told them he put their riches in a pot of gold at the end of the rainbow, and the story goes the couple is still looking for the end of the rainbow to this day.
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