Conjoined twins are among the rarest of human beings. There are probably fewer than a dozen adult pairs living in the world today. Only a few hundred pairs of conjoined twins are born in the whole world each year - they appear about once in every 100,000 births - but more than half of them are stillborn, and one in three live for only a few days. Of those who survive, a very small number will be selected for separation surgery. But as there are few hospitals with the skills and experience to perform this kind of surgery, separation is still a very unusual event. The harrowing decisions which surgeons have to make when faced with conjoined twins have been highlighted by the recent case in Manchester, England. Separating conjoined twins is not only technically challenging; it can involves life and death decisions about whether one twin should be sacrificed in the hope of saving the other. But "sacrifice surgery" has a poor record of success, and the Manchester case is the latest round