I've learned a hard and difficult lesson considering the childhood environment that I was brought up in - not to discriminate against any plant or creature.

BBC: Norfolk - Chris Skinner's wildlife haven

Still, the most important lesson from the trial may be that it is incredibly difficult to tease out the long-term effects of a medicine.

FORBES: Wyeth Lucks Out On Hormone Study

They say they learned that difficult lesson in two of the last three years, when hail destroyed most of their crops.

CNN: farmer plows

It's not difficult to find a lesson in the stones for those of us trying to simplify our sprawling lives.

CNN: The seven stones of simpler living

But Painter no doubt learned a hard lesson about how difficult it is to force change in the auto industry.

FORBES: TrueCar.com Founder Gets A Do-Over After Almost Killing His Company

But to outsiders the lesson seemed to be that selling to online consumers was actually a difficult, unglamorous business.

BBC: Dot.com to Dot.bomb