When the price of gasoline rises, for example, it encourages profit-seeking firms to take several actions: expand exploration for oil reserves, drill for more oil, invest in more pipelines and oil tankers to bring the oil to plants where it can be refined into gasoline, build new oil refineries, purchase additional pipelines and trucks to ship the gasoline to gas stations, and open more gas stations or keep existing gas stations open longer hours.
Economists call this positive relationship between price and quantity supplied—that a higher price leads to a higher quantity supplied and a lower price leads to a lower quantity supplied—the law of supply. The law of supply assumes that all other variables that affect supply are held constant.