. The most visible example might be a case of conjoined twins sharing a brain, says Kramer, but even regular twins could have shared organs without realising it. During early development, cells can be passed between twins or triplets. Once considered a rare occurrence, we now know it is surprisingly common. Around 8% of non-identical twins and 21% of triplets, for example, have not one, but two blood groups: one produced by their own cells, and one produced by “alien” cells absorbed from their twin. They are, in other words, a chimera – a fusion of two bodies – and it may occur in many organs, including the brain