What ingredient prevents an egg from stiff peaks?

The presence of fat, such as that from egg yolks or any added fats like oil or butter, can prevent egg whites from achieving stiff peaks. Even small amounts of fat can interfere with the proteins in egg whites from properly whipping. Additionally, any residue of grease or oils on mixing equipment can also hinder the whipping process. For optimal results, it's essential to use clean, dry equipment and separate the egg whites from yolks carefully.