This is the 2544th most frequent Spanish word.
cae
"Falls" (third-person singular present of "caer").
Here, 'cae' is used to describe the downward motion of rain, indicating its physical descent.
La lluvia cae suavemente sobre los campos.
The rain falls softly over the fields.
In this context, 'cae' metaphorically represents committing a mistake or being led into an undesirable situation.
Si no tiene cuidado, cae en el error con facilidad.
If he is not careful, he easily falls into error.
Here, 'cae' is utilized metaphorically to describe the sudden appearance or manifestation of a smile.