The law of demand

All else equal, as the price of a good rises the quantity people want to buy falls — and as price falls, quantity demanded rises. That inverse relationship is why demand curves slope downward.

• A price change moves you along the demand curve; a change in tastes, income, or the price of related goods shifts the whole curve.
• The substitution effect: when something gets pricier, buyers switch to alternatives.
• The income effect: a higher price makes buyers effectively poorer, so they buy less.

A coffee shop raises a latte from $4 to $5. The next morning, a handful of regulars switch to filter coffee or skip the second cup — quantity demanded falls. Nothing about the customers changed; only the price did, so we slide up the same demand curve rather than shifting it. If instead a viral post made lattes trendy, the whole curve would shift right and more would sell at every price.

Always separate a movement along the curve (caused by the good's own price) from a shift of the curve (caused by anything else). Mixing them up is the single most common mistake in intro microeconomics.