Original: Sulfur contamination of iron and steel make them "red short", so that at red heat they become "crumbly" instead of "plastic".
Simplified: This is called "work-hardening". The next step is to "draw the temper" or "temper" the steel, so that it will not break like glass. After the "heat-treat" and "temper", stones are used to grind the blade edge to make it sharp.