Question: A woman has four children conceived naturally with her husband, but a reliable genetics lab test concludes she could not be their biological mother. How is this possible?

Answer: She is one of some 30 documented cases of a human "tetragametic chimera," where two non-identical twins combine in the womb at an early stage to form a single organism, says online "Wikipedia: The Free Encyclopedia." Here the embryo is formed of four gametes—two eggs and two sperm—with two genetically distinct population of cells. The resulting chimera can possess organs with different sets of chromosomes, such as a liver composed of cells with one set of chromosomes and a kidney of cells with another set.

It is hard to pin down just how many human chimeras there are since they can only be detected with DNA testing, which is rare, says "New Scientist" magazine. In one case, when patient Jane needed a kidney transplant, her family underwent blood tests to find a suitable tissue-type donor. Back came the news that two of her three presumed sons couldn't be hers! After lengthy investigation it was concluded that Jane is a chimera, a mixture of two non-identical-twin sisters that had fused in the womb.

Bizarrely, some researchers now believe we're all chimeras of a sort, not pure-bred individuals but "teeming with cells from our mothers, maybe even from grandparents and siblings," says "New Scientist." During pregnancy, the blood of mother and fetus are kept separate, but some cells manage to slip through. "So a single person can be a veritable menagerie of different cell types from different generations."

Question: If you were to draw a centerline down Leonardo da Vinci's painting of the Mona Lisa, what eye-catching body part would it pass through? See where this might be heading?

Answer: When vision researcher Christopher Tyler did this with face-on portraits from 265 artists spanning six centuries, he was looking to find subtle differences in the two facial halves, due to right-brain/left-brain differences in each artist. He found something totally surprising instead: In drawing the dividing centerlines, he noticed how often these ran right through one of the eyes of the subject, as with the Mona Lisa's left eye at horizontal dead center! ("American Psychological Association Monitor")