A joke that Freud tells in Jokes And Their Relation To The Unconscious illustrates well the point that Lacan is making about how desire differs from both demand (what you ask for) and need (what you cannot do without). The joke goes:
“An impoverished individual borrowed 25 florins from a prosperous acquaintance, with many asseverations of his necessitous circumstances. The very same day his benefactor met him again in a restaurant with a plate of salmon mayonnaise in front of him. The benefactor reproached him: “What? You borrow money from me and then order yourself salmon mayonnaise? Is that what you’ve used my money for?” “I don’t understand you”, replied the object of the attack; “if I haven’t any money I can’t eat salmon mayonnaise, and if I have some money I mustn’t eat salmon mayonnaise. Well, then, when am I to eat salmon mayonnaise?” (SE VIII, 49-50).
Commenting on this joke in Seminar V, Lacan says:
“[The joke shows]… the relationship between the signifier and desire, and the fact that desire has profoundly changed its accent, has been subverted, has been made ambiguous, by its passage through the paths of the signifier. Let us be clear what that means. It is always in the name of a certain register that makes the Other intervene beyond the one making the demand, that any satisfaction is accorded, and precisely this profoundly perverts the system of demand and of the response to demand. “Clothe the naked, feed the hungry, visit the sick….” I do not need to remind you of the seven or eight or nine works of mercy. It is striking enough in their very expression, that in clothing the naked, one could say that if the demand were something that should be directly sustained in its fullness, why not clothe the naked man or woman at Christian Dior’s?…The same goes for feeding the hungry. Why not let them get drunk?” (Seminar V, 04.12.057., p.6).
To paraphrase Lacan’s point, the joke shows that desire is like a ‘perverted’ form of need by the very process of expressing that need in the form of a demand. The young benefactor obviously needs food, but his hunger can obviously be satisfied by something less than the most expensive and elaborate meals. So we find desire in what ‘pushes beyond’ need in the very expression of a demand.