Hijabbering Kid

Today I stopped at a convenience store with my five year-old son. The woman working there was wearing a hijab. Being the shy boy that he is, he immediately asked her, "Why are you wearing that mask?" (I immediately wished I had one) She replied very nicely that it was part of her religion and that she wears it for the same reason he wears a shirt.

When we got back in the car he was still unsatisfied with that answer, so I explained that she was wearing it so no one would recognize her when she was taking flight lessons.

Actually, I didn't have a decent explanation for him. He doesn't understand concepts like modesty (he REALLY doesn't understand that one), or even religion. If anyone has any suggestions on this, I'd love to hear them.