Spend a fixed income on two goods and find the bundle that makes you best off.
Understand a budget as a real boundary on choice — why getting more of one good means giving up another, and where the best affordable bundle sits.
Raise the price of good A.
Before you act, predict what will happen. Which way does the budget line move — and does the best bundle stay put? Then do it — did your prediction match what happened?
The budget line and your best bundle
Every point on the blue line spends all your income; utility peaks where an indifference curve just touches it.