There have been great inventions throughout the course of human history: The wheel, the printing press, the ball, the automobile, the internet. Each one of these inventions has changed the course of human civilization and the way we live.
Now there is a new innovation. It probably isn't as good as the wheel or the internet. It might not even be as good as the ball, but it's still really good; so don't stop reading, please.
This innovation is the gray water recycling system. It uses your smelly, old shower water and converts into brand-new, sanitized toilet water. I don't know why you need to sanitize toilet water, considering what it's going to be used for. Maybe it's for people who have a dog and are lazy and they don't want their pet drinking dirty shower water straight from the toilet?
Some of these systems reuse the water for washing machines. I can see someone wanting sanitized water for that. My assumption is that it is just easier to put a sanitizer in each model and forget about it instead of making two different types of models. I might have to write a letter to David Feldman or Cecil Adams for an answer to that question.
You can also modify your sink to fill your toilet and reuse your shaving or toothbrush water as well. These systems cut down on water usage by about one-third and usually end up paying for themselves after a few years. The only downside: You have to change a filter.