The one about the black baby...

Sandy and Thor were an extremely liberal, though not especially bright, white couple. Wanting to begin a family, they decided they wanted to have a black baby.

Nine months later, the fruit of their labor was born: a lovely white girl.

Pleased but disappointed, Thor decided to ask a black man at work why he thought he couldn't make a black baby.

Realizing that Thor was somewhat sluggish, the fellow took him aside and asked, "Is your willy at least a 12" long?"

Thor had to admit that it was not.

"And is it at least 4" wide?"

"Well, man, there's your problem!" Black guy slapping Thor on the back. "You let in too much light!"