If you say something to me with the aim of getting me to do something for you, such as ʻkiss me!ʼ or ʻget out of my daylight!ʼ, or to give you some object, as in ʻpass the salt!ʼ, the exchange commodity is strictly non-verbal: what is being demanded is an object or an action, and language is brought into help the process along. This is an exchange of goods-&-services. But if you say something to me with the aim of getting me to tell you something, as in ʻis it Tuesday?ʼ or ʻWhen did you last see your father?ʼ, what is being demanded is information: language is the end as well as the means, and the only answer expected is a verbal one [...] These two variables, when token together, define the four primary speech functions of OFFER, COMMAND, STATEMENT and QUESTION. These, in turn, are matched by a set of desired responses: accepting an offer, carrying out a command, acknowledging a statement, and answering a question.
- Halliday (1985), a pag.68