They say that if you put a frog into a pot of boiling
it will leap out right away to escape the danger.
But, if you put a frog in a kettle that is filled with
water that is cool and pleasant,
and then you gradually heat the kettle until it starts boiling,
the frog will not become aware of the threat until it is too late.
The frog's survival instincts are geared towards detecting sudden changes.
This is a story that is used to illustrate how people
might get themselves into terrible trouble.
This parable is often used to illustrate how humans have to be careful
to watch slowly changing trends in the environment, not just the sudden
changes. Its a warning to keep us paying attention not just to obvious
threats but to more slowly developing ones.