Fresh water is a scarce resource everywhere in the world. The amount of water wasted from washing our hands or brushing our teeth is enormous. The Eco Bath by Jang Woo-seok takes grey water from the sink and uses it to flush the toilet.
What a great idea! Take the grey water (which is relatively clean) from washing your hands, combine it with fresh water and use that mixture to flush the toilet.
The two tank design holds both fresh water and grey water that was used in the sink to flush the toilet.