What makes a substance smell like rotten eggs when burning?

A substance smells like rotten eggs when burning primarily due to the presence of hydrogen sulfide (H₂S) gas, which has a characteristic odor similar to that of rotten eggs. This gas can be released during the combustion of organic materials, particularly those containing sulfur compounds. When these substances are heated, sulfur can react with hydrogen or other elements, producing hydrogen sulfide and contributing to the unpleasant smell. Common sources include certain types of fossil fuels and decaying organic matter.