Out of the mouths...

...of stepsons.

"Why does bottled water have an expiry date?"

February 20, 2011. There it was.

Google was not my friend. Suggested reasons:

• Chemicals from the bottle leach into the water
• It's planned obsolescence, to increase the water company's profits
  
• The law says that every consumable product must show an expiry date, so the companies just pick one at random
  
• The water changes taste over time
• The water companies slip fluoride into the water, which eventually expires

And my personal fave:

• "Water which is not flowing can go stagnant. Bacteria and even small worms can appear."

Plausible answers are welcome. I must admit that the leaching explanation sounded convincing.

In the meantime, here's The Economist, if you can believe it:

The success of bottled water is in many ways one of capitalism’s greatest mysteries. Studies show consistently that tap water is purer than many bottled waters—not including those that contain only tap water, which by some estimates is 40% of the total by volume.

OK, then. All of the above.

[H/t Danny MacKinnon].