Why does the streak test work only on minerals softer than porcelain?

The streak test works only on minerals softer than porcelain because it relies on the ability of a mineral to leave a colored mark or powder on a porcelain plate. If a mineral is harder than porcelain, it won't leave a streak, as it cannot scratch the surface. Porcelain typically has a hardness of about 6 on the Mohs scale, so only minerals with a hardness less than this can effectively produce a streak. This property helps in identifying minerals based on their color and texture.