Buildings, bridges, cars, power grids, data centers — almost everything starts with two things: steel smelted from ore in a furnace, and aluminum “pulled apart with electricity.” Each year the world makes nearly 1,900 million tons of steel and about 72 million tons of aluminum — but this “smelting” step alone emits about 8% of all global carbon and uses more electricity than many countries combined. This lesson walks through how civilization's most basic metals are actually made, why China controls over half the world's smelting, and why “changing the furnace” is becoming the biggest battlefield in heavy industry.