Supply & Demand: Why Prices Move

Why does anything cost what it costs?

Why is a mango sometimes HK$10 and sometimes HK$30? Why does an umbrella suddenly get expensive when it rains? Why are concert tickets sold out in 5 minutes but maths textbooks sit on shelves forever?

All of it comes from two simple ideas: supply and demand.

Demand: how much people WANT it

Demand is how many people want to buy something, at a given price.

The key rule:

When price goes up, demand goes down. When price goes down, demand goes up.

This is just common sense. If mangoes cost HK$1, you might buy 10. If they cost HK$100, you’d probably buy zero. Same mango, totally different decision.

Supply: how much there IS

Supply is how much of the thing is available to sell.

• Lots of mangoes at the market = high supply.
• A rare limited-edition card = very low supply.

Where they meet: the price

A market finds a price that roughly matches supply and demand.

• High demand + low supply = high price. (Concert tickets, rare sneakers.)
• Low demand + high supply = low price. (Last year’s toys in a clearance bin.)

When something’s popular (high demand) and hard to make (low supply), it gets expensive fast. When nobody wants it and there’s tons of it, the price falls until someone gives in.

Try it: your lemonade stand

You’re selling cups of lemonade. It costs you HK$2 to make each one. How much should you charge?

Move the price slider. Watch what happens to cups sold and profit.

Things to try:

• HK$1 per cup. Everyone buys, but do you actually make money?
• HK$5. Good balance — many customers, decent profit per cup.
• HK$15. Very few customers — maybe you’d earn more per cup but barely anyone buys.
• HK$20. Almost nobody buys. You walk home with lemons.

The “sweet spot” is somewhere in the middle where:

• Enough people still want to buy (decent demand)
• Price is well above your cost (decent profit per cup)

The Umbrella Story

Here’s a classic example of supply and demand in real life.

A shopkeeper in Hong Kong sells umbrellas. On a sunny day, he might sell one umbrella for HK$50. When it starts raining unexpectedly, suddenly hundreds of people want an umbrella. He has only a limited number.

What does he do?

• Option A: Keep the price at HK$50. The umbrellas sell out in 10 minutes.
• Option B: Raise the price to HK$100. He sells fewer but earns more per umbrella.

If he doesn’t raise the price, the first 50 lucky customers get umbrellas and everyone else gets wet. If he does raise the price, the umbrellas last longer, and only the people who really need one (or can pay) buy.

Neither is “right” — but both are examples of supply and demand at work.

Who wins, who loses?

Supply and demand isn’t always “fair.” A few examples of its effect:

• Rental housing in Hong Kong is expensive because millions want to live here (high demand) but there’s limited land (low supply). Many people can barely afford rent.
• Taxis during a rainstorm vanish because demand jumps but the number of taxis stays the same.
• Face masks in early 2020 cost 10× normal because everyone wanted them at once.

When you see a price that feels unfair, ask: is the demand huge, the supply tiny, or both? You’ll usually find your answer.

Practice