What enables us to tolerate an interruption and then return to our previous thoughts? This must engage the agents that control our short-term memories. It is important also to recognize that many interruptions come not only from outside, but also from inside the mind. For example, all but the simplest discourses make interruptions in the trains of thought they start. Consider this sentence:
The thief who took the moon moved it to Paris.
We can regard this as expressing one thought that is interrupted by another. The principal intention of the speaker is to express this Trans-frame:
The thief moved the moon (from ?) to Paris. Actor Trans Object Origin Destination The speaker, realizing that the listener may not know who the thief was, interrupts the main sentence with a relative clause — who took the moon — to further describe that Actor thief. As it happens, this interrupting clause also has the form of a Trans-frame — so now the language-agency must deal with two such frames at once.
Who took the moon (from ?) (to?) Actor Trans Object Origin Destination English tends to use certain wh words, like which and who, to interrupt a listener's language-agency and cause its short-term memories to temporarily store away some of their present pronome assignments. This provides the language-agency with more capacity to understand the interrupting phrase. In the case of the moon sentence, the word who instructs the listener to prepare to elaborate the description of the Actor thief. Once this is done, the language-agency can re-member its previous state in the process of understanding the main sentence. We can often tell when to use an interruption process even though the initial wh word is missing; however, this doesn't always work so well:
The cotton clothing is made of is grown in the south.
This sentence is confusing because the reader tends to treat the word cotton in cotton clothing as an adjective that modifies clothing, when the writer meant it as a noun. The same sentence is easier to understand when set in a larger context:
Where do people grow the cotton that is used to make clothing? --- The cotton clothing is made of is grown in the south.
The first sentence activates the noun sense of cotton and asks a question about that subject. Now a question is really a sort of command: it makes the reader focus attention on a certain subject. Here, it prepares the reader to add more structure to the representation of the cotton noun, so there is less need for an explicit interruption signal. Still, it is very curious how rarely we bother to use any signal at all for marking the end of an interrupting phrase. We never say a word that means un-who. Evidently, we're usually ready to assume that the interrupting phrase is complete.