Early humans gathered fruits, nuts and herbs for survival and invented fire for cooking. The discovery that meat could be cooked over fire led to experimentation with cooking methods and use of shells and stones as early cooking pots. Clay pots then allowed for variety in cookware shapes and sizes, and developments in metallurgy introduced steel, stainless steel and aluminum pots and pans. Modern kitchens now utilize glass cookware safe for microwave and stovetop use.