Twin

Even identical twins with the same genes can differ in handedness: one may be left-handed and the other right-handed. Such discordance in twin studies shows that although genes influence handedness, they do not determine it alone.

In mammal births, a round lump of flesh covered in skin and fur sometimes appears, called 'amorphous globosus.' It is a twin that failed to develop and survives by parasitizing the other's blood. In cattle, it occurs about once in every 3,500 pregnancies.

Some twins spontaneously create a unique language only they understand, a phenomenon called cryptophasia. The most famous case is June and Jennifer Gibbons, the 'Silent Twins,' whose language was so distinct that even their parents could not comprehend it.