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A video showing a man cracking a giant egg to reveal another fully formed egg inside, may be enough to scramble your brain.

YouTube user Elman511 cracked open the over-sized egg that was laid by one of his hens, exclaiming: 'The big egg has another normal egg in it!'

While the instance is extremely rare, it's caused when the hen is in the process of forming an egg in her oviduct and another oocyte – the ovum or egg cell that becomes the yolk of the egg – is released too soon.

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Egg-traordinary: US YouTube user Elman511 cracked open an over-sized egg (shown on the left) that was laid by one of his hens, exclaiming: 'The big egg has another normal egg in it!'

In the video, Elman511 cracks the egg, which is double the size of a normal one, to find a standard-sized egg inside, along with yolk and white.

He then cracks the smaller egg, which also has a yolk and white inside.

'That's the end of the nesting dolls of egg…an egg inside of an egg… crazy man,' he said, before noting that the yolks look rich.

Egg-citing: In the video, 'Elman511' cracks the egg (pictured), which is double the size of a normal one, to find a standard-sized egg inside, along with yolk and white. The unusual occurrence is down to a counter-peristalsis contraction

Egg-ceptional! Amazing moment man finds an egg within an egg

The unusual occurrence is called a counter-peristalsis contraction.

A hen releases an oocyte from her left ovary into her oviduct every 18 to 26 hours, Back Yard Poultry Mag explained. 

Usually, the oocyte travels down the oviduct, adding layers of the egg until it's laid through the hen's vent.

But in a counter-peristalsis contraction, a second oocyte is released by the ovary before the first egg has completed its journey.

When the second oocyte's released, the first is in the eggshell-gland portion of the oviduct, were the shell is made.

It's thought that a shock, or stress causes a contraction, forcing first egg to reverse its course.

This means that the first egg is added to the second oocyte, which then travels down the oviduct and is covered in a shell, creating a giant egg around the first one.

The phenomenon has been mentioned as far back as 1250 AD by the Dominican friar and polymath Albertus Magnus in his book De Animalibus.

Ask the egg-spert: In a counter-peristalsis contraction, a second oocyte is released by the ovary before the first egg has completed its journey. This results in an egg with another made inside it, producing two yolks (pictured)