When you’re chaperoning a school trip, you notice things.  Annoying things.  Shown below is a sign at Philadelphia’s Fairmount Water Works Interpretive Center that makes the claim that all the water molecules on Earth are never, ever destroyed — they are immortal entities. If you teach biology, you’ll be instantly outraged, especially when you think of the hundreds of thousands (millions?) of kids who’ve absorbed the contents of this signage as fact.  Details below the image, but see if you can figure out the flaw before you jump.

The sign is wrong partly because of photosynthesis, which usually involves the splitting of water molecules (to generated electrons).  That little trick evolved about 3,500,000,000 years ago, so I’d wager that most if not all the water originally present on the planet has been replaced by new molecules produced from combustion (including respiration). That’s just a guess, though…I couldn’t find a calculation on the internet.  Download this photograph and use in your lectures to introduce the ideas of photosynthesis and respiration.  If you lecture on science center signage, you can use this to highlight the value of getting a few scientists to proof the graphics.  Or a few 7th graders.