AN OBJECT LESSON IN NATURAL HISTORY

This is a healthy frog in its natural habitat (freshwater):

This is chlorine (Cl, atomic number 17), used as a chemical weapon during the Great War (the Germans threw chlorine grenades at the British and French intrenchments):

These are chlorine tablets, used by humans to purify freshwater:

This is a chlorine dispenser, used to purify freshwater in swimming pools. It contains chlorine tablets:

And this what happens when a frog meets a chlorine dispenser:

A dead frog.