60%-71% represents the water percentage found in a mature human body, and on the Earth’s surface.
Entering the gallery, the visitor will find himself facing a big transparent pool filled completely with crystal clean water. Then he/she will be encouraged to throw his/her garbage inside.
There are two possible outcomes, either they do (each object thrown in the water will cause some of it to spill on the floor), either they don't (contemplating on why they find it hard to do so), both working towards enforcing the same metaphor.
The tank will not be cleaned throughout the exhibition.