Las manos o mis manos?

Question

I encountered the following question during an exercise at A1 level:

"Yo ______ mis manos en el agua. (I plunge my hands in the water.)"

It is my understanding that mi/mis should not be used with body parts or clothing. Could you please advise?

Thanks

Answer

Hola Martin,
We do tend to use the definite articles (el, la, los, las) instead of the possessives (mi, tu, sus...) when talking about body parts, as explained in this A1 lesson. 
This example that you encountered in one of the quizzes is testing the verb (sumergir) in the present tense:

Yo sumerjo mis manos en el agua.
(I plunge my hands in the water.)

We could have used "las manos" here as well, as this sentence reflects what we are explaining in that lesson. However, the verb "sumergir" (to plunge/ submerge) is a bit formal, and the use of the possessives here click nicely. It gives the whole sentence a more formal effect. 
If we were using a more basic, routine/hygiene verb, the the possessive (although grammatically correct) would sound a bit odd. For example:
1. Ella se lava las manos con jabón. (natural way to say: She washes her hands with soap.)
2. Ella se lava sus manos con jabón. (very unnatural way to say: She washes her hands with soap.)
I hope this helped.
Saludos.
Inma