Gender agreement of examples

Question

I don’t understand why  “Je me suis bien amusée.” adds the “e” for the feminine subject but “Je me suis lavé les dents.” does not when both speakers are female.

Melisa McCurley · Answer

It's been a while. Does no one know the answer?

Alan Graham · Answer

It's because there is a direct object "les dents". This question has been asked a lot, but so far the lessons have not been updated to explain this. Look at the answer to Gabrielle's question at the end of the Q&A here:

Agreeing past participle with subject's gender and number with (+ être) verbs in the compound past in French (Le Passé Composé)

Melisa McCurley · Answer

Thanks so much for pointing me to the answer, Alan. I understand now but it's getting complicated! :) It would surely help if they'd add a note about it to both those lessons. I did read through all the questions on the lesson I asked about and didn't see an answer there. It should really be part of the lesson, or have a link to a lesson discussing it.