How does washing your face and "anointing your head" hide the fact that you are fasting?
But you, when you fast, anoint your head, and wash your face; (Matthew 6:17)
It wouldn't, except that people might expect you to be like "The hypocrites":
They neglect their appearance, so that they may appear to others to be fasting.
You might expect this dialogue:
Hey Joe, you OK these days? Man, you gotta take care of yourself, you know, shower once in a while, wash the hair, right?
Hey man, lay off, I'm fasting, you know?
Oh, hey, fasting! Well nothing wrong with that, man. Good on you!
And "they have already received their reward". If you look after your appearance, only God, "who sees what is hidden" and will repay you.