So the tortoise is caught, much like the grammar concept, and placed in its new environment. It is safe, and secure and then one turns away and gets distracted. The tortoise is gone, tucked under who know what piece of furniture, and no matter how many times it is found and brought back to the center of the room it is always moving. The fable is reveled true.
Just like the tortoise of legend concept in the human brain are constantly moving and if one fails to refresh both knowledge and practical skill on a regular basis the concept will be discovered only in its absence. A classic example is the that/which issue.
According to exhaustive studies (and boy is this author exhausted) there are three basic rules.
1) THAT: Used without a comma to introduce an essential clause (one critical to the meaning of the sentence).
2)THAT: Used without a comma to introduce a dependent clause, the absence of which would make the sentence look or sound awkward.
3)WHICH:Used with a comma to introduce a non-restrictive clause.
Hopeful this particular tortoise stays around a while.