Humans have been fascinated by twins for centuries, but while seeing double is bound to make anyone look twice, it’s the science behind twins that makes them truly special. In a recent YouTube video, the team All Time 10s compiled a list of 10 of the most strange facts about twins that will leave you speechless.

The defining feature of being a twin is sharing a birthdate, but in the advent of modern medicine, this is no longer always a given. As explained in the video, sometimes one twin may be born prematurely while the other twin is left in the womb as to further mature. The result? “Twins” who can actually be born months apart.

In addition, twins usually have the same parentage, but once again, as explained in the video, this is not always true. Due to a strange phenomenon known as heteropaternal superfecundation, if a woman has two different eggs fertilized by two different men within the same week she can give birth to twins with different fathers. This can also occur during in vitro fertilization, most often due to a mixup in sperm. As a result, siblings born at the same time to the same mother can actually even have different ethnicities.

Other fascinating features of some of the world’s most rare twins include the ability to read each other’s mind, invention of a language only understood by them, and the strange abilities for twins to develop similar personalities and hobbies, despite being separated at birth. See the video below to discover even more facts about twins.

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