Quirk of Spanish. If you say:
- "... lavar las manos?" it means "Where can we wash the hands" since there is nothing to indicate that we speak about our hands.
- "... lavar nuestro manos?" it means "Where can we wash our hands". However, it sounds like our hands are not part of our body anymore.
That's why we say "... lavarnos las manos?", which means "We wash ourselves, and specifically only our hands.
As AmineHadji1 explained, using the reflexive form of llevarse indicates that we are washing our own hands. When used this way, it can't be "the hands" unless you expand that to something like "the hands of ours," which would sound really odd in English.
To put it more simply, Spanish uses the definite article when referring to one's body parts or articles of personal clothing.
By the way, DL didn't use to follow this rule. It only started following it recently (Last 2-3 months?)
Yes, but you can wash someone else's hands and still use the definite article. With the reflexive form of the verb there is no ambiguity.
La madre lavó las manos = "the mother washed the hands (of someone)"
La madre se lavó las manos = "the mother washed her (own) hands"