Fleas are incredibly powerful little creatures. They jump with acceleration of around 100G – far greater than that of a space shuttle lifting off.
If a flea is put in a jar, it will leap straight out of its limited environment. If the jar is closed with a lid, the flea will jump a few times and bash its head off the lid. Not liking being bashed, the flea who loves jumping will modify its jumps so that it doesn't whack the lid any more. Most people (and fleas) would agree that this is a good strategy.
Guess what happens when the lid is removed from the jar.
The flea jumps...to below the height where the lid used to be. It will never jump higher than this again. Without some sort of external interference, the flea will never know life outside of the jar again. It has came to accept a limit that it will never test again.
This possible urban myth vividly illustrates a common phenomenon I see a lot in my work.
Somebody gets hurt. They limit their actions accordingly to allow recovery. The person spills the limit over into unnecessary activities. They pollute the rest of their mind with thoughts of what they can't do. The direct huge amounts of attention to a tiny limitation. Sometimes, they don't even notice when it is gone. Sometimes they need a flee friend to tell them when the lid has been lifted.
Having jumped 14cm to avoid a 15cm ceiling for so long, what would happen if you slowly edged, millimetre by millimetre up to the 15 mark? Is 14.9 fine? Does your head brush the ceiling when you jump exactly 15?
Wow – it didn't! Do it a few times. Are you happy that it's safe? How about 15.1? Fine? 15.2? Good.
As quickly as you comfortably can, you can forget the lid. You can leave the jar. You can be free as a flea.