When do you use definite article with parts of the body?

Question

Tomó uno de los arándanos entre los dedos y lo llevó a su boca
I put la boca but I dont see what the rule is here.
Gracias

Silvia Píriz · Accepted Answer

Hola David L.
Good question! In Spanish, body parts are often used with the definite article (la boca, las manos, los ojos, etc.) instead of a possessive adjective when it's already clear whose body part we're are talking about:

Me duele la cabeza. (My head hurts.)
Se lavó las manos. (He washed his hands.)
Lo llevó a la boca. (He brought it to his mouth.)

In your example, however, su boca is also perfectly natural because the sentence explicitly refers to the cranberry belonging to a particular person. Both la boca and su boca are possible here, although la boca is generally the more idiomatic choice in Spanish.
Saludos
Silvia