Where is the joke?

8. On the roadside, a blind beggar was begging in the street wearing sunglasses. A drunk came up and felt sorry for him, so he threw him a hundred dollars. After walking for a while, the drunk turned around and happened to see the blind man with his back to the sun to distinguish the authenticity of a hundred-dollar bill. The drunk came over and took back the money and said, "You don't want to live, how dare you lie to me!" " "

The blind beggar looked aggrieved and said, "Brother, I'm really sorry. I'm looking for a friend. He was blind and went to the toilet. In fact, I am dumb. " "Oh, that's right!" So the drunk dropped the money and staggered away again. ...