Let's build the strawberry market of one stall, in kilos per day. On the SELLERS' side: below €5/kg, the price does not cover costs, and nobody brings out their crates — supply is nil. Above it, each extra euro makes selling profitable for more producers and brings out 4 more kg. So we write down exactly what we have just said: 4 kg per euro ABOVE €5, that is Qs = 4 × (P − 5). Expanding gives the form usually met, Qs = 4·P − 20 — the “−20” means nothing on its own, it is only the 4 × 5 of the threshold. On the BUYERS' side: if strawberries were free, 100 kg would go in a day; each euro of price puts 8 kg off. Hence Qd = 100 − 8·P, which falls to 0 at €12.5/kg — beyond that, not a single buyer. These four numbers are invented for the example, but each has a meaning: two thresholds (€5 and €12.5) and two sensitivities (4 and 8 kg per euro). One word finally about the usual presentation, in case you meet this market elsewhere as a graph: the convention is to put quantity on the horizontal axis and price on the vertical one — demand slopes down from left to right, supply slopes up, and equilibrium is where they cross. It is disconcerting, since it is the price that commands quantities, but the usage is firmly established.

One single price makes the quantity demanded equal the quantity supplied. Set the equality: 100 − 8·P = 4·P − 20. Add 8·P to both sides: 100 = 12·P − 20. Add 20 to both sides: 120 = 12·P. So P* = €10/kg. That leaves HOW MUCH changes hands at that price: Qd = 100 − 80 = 20 kg, and Qs = 40 − 20 = 20 kg — the two coincide, a good sign. So a market's outcome is not a number but a PAIR: (P* = €10/kg; Q* = 20 kg). They are called the equilibrium price and the equilibrium quantity.

Here is the heart of the chapter. If the price goes from €10 to €11, the two relations have not changed one jot: you are simply reading another of their lines. That is SLIDING ALONG the curves. But if the frost destroys half the harvest, then at EVERY price there are half as many strawberries to sell: the supply relation itself is replaced, the curve SHIFTS, and the equilibrium has to be recomputed. That is how the apparent contradiction of the poll unravels: the frost lowers supply (a shift) while the rule “higher price ⇒ larger quantity supplied” goes on holding on the new curve (a slide). What shifts the curves? Never the price of the good itself — everything else: incomes, tastes, the price of other fruit, the weather, costs, taxes, the number of buyers or sellers (interpretation, point 2).

That leaves WHY the price would settle at €10. Try €8: buyers want 100 − 64 = 36 kg, and sellers offer only 32 − 20 = 12 kg. 24 kg are missing: that is a SHORTAGE, and disappointed buyers agree to pay more rather than go home empty-handed. Try €12: buyers want only 4 kg while sellers offer 28. 24 kg are left on the stall: that is a SURPLUS, and the seller cuts the price at the end of the market rather than throw them away. In both cases, the pressure pushes towards €10. One caveat, though: this convergence is a plausible mechanism, not a guaranteed law — the third card in the limits shows a worked case where the price never settles. Click each term: