save the last DROP!
The water below represents virtually all the water in the world, but some segment has been rendered unusable (unsafe, undrinkable) as a result of the dumping of pollutants into it. The drop falling from above represents the only source of fresh water to the world, but it may also be exposed to the similar harmful human activities towards the environment, thus the drop of will eventually be polluted or destroyed.
This therefore conveys the seriousness of the situation and the fact that we (students, governments, civil society, international development agencies, and humankind) have to act, and quickly too, otherwise we might wake up to realise sooner than later that we have sentenced the entire human race to death by our actions and inactions that pollute the water bodies in the world. Hence, the clarion call to humanity, save the last DROP!