أَمْطَرَتِ السَّمَاءُ نَبَاتًا
“The sky rained plants,” meaning rain occurred that would bring vegetation.
The word “plants” signifies the product of rain, yet in the context of raining it indicates the water that brings about that product. Here the effect's name is transferred to its cause.
“Raʿaynā al-ghayth” — cause named, effect intended.
“Amṭarat al-samāʾ nabātan” — effect named, cause intended.
The distinction depends on which side supplies the expression and which side supplies its contextual meaning, rather than on a label memorized without analysis.