Supply and Demand
Why does the price of something go up or down? Often the answer is supply and demand.
Supply is how much of something is for sale. Demand is how much people want to buy it. The price depends on how the two fit together.
Imagine a stand that sells lemonade. On a cool day, few people feel thirsty, so demand is low. The stand has plenty of lemonade, so supply is high. To get people to buy, the owner may lower the price.
Now imagine a hot day. Many thirsty people want a cold drink, so demand rises. If the stand has only a little lemonade left, supply is low. The owner can raise the price, and people may still pay it.
The same pattern shows up in stores everywhere. When a toy is hard to find and everyone wants one, its price may rise. When a store has too many winter coats left in spring, it may put them on sale.
Prices do not change by magic. They change when supply, demand, or both change.