Collecting rainwater is an old idea that goes back as far as ancient India and Rome. In modern society, it's a clever, cheap way to gather a little excess water for your garden or your lawn while also doing your part for the environment. But that's not the case in Colorado, where so-called "rain harvesting" is illegal. In fact, it's not just against the law, it's written into the state constitution.

The reason for this rather confounding legal situation is a concept called "prior appropriation." In most Western states, water rights are not contingent on who owns the land but are instead apportioned as part of a historical first-come, first-served policy. Essentially, back when the Western states were originally settled, the first person to use a particular body of water for a beneficial use (i.e., farming, mining, household use) was granted the right to that particular body of water.

If they didn't need all the water, someone else could be granted secondary use, and on and on down the chain until all the water was spoken for. Furthermore, the water rights are granted in perpetuity, meaning a farmer today can be using the water rights passed down to them from their great-great-grandfather who settled the land in the 1800s.