When the two angels arrive in Sodom in Genesis 19, Lot seems to immediately recognize them as such. He bows down to them, calls them "My lords", and invites them into "your servant's house." When the men of the city surround the house, though, they call out, "Where are the men who came to you tonight?" Of course, angels in the Bible are sometimes ambiguously referred to as men.
Does Lot recognize that they are angels or are his actions just customary hospitality? If he does, do the men of the city also recognize their nature? And if not, why not when Lot identifies them so easily?