A typical chicken egg weighs about two ounces or 50 to 60 grams. But a YouTube user named Sean Wilson found a very large egg, as seen in the video below (also available here). He weighed it to find that this super egg weighed more than three times the typical amount, at over six and a quarter ounces (181 grams). When he cracked it open, the explanation became clear: there was an egg, still in shell, inside his egg.

While double-yolk eggs are (while atypical) not uncommon, something like the above feels impossible. But the video above isn’t a hoax. It’s a rare phenomenon called “ovum a ova,” per the BBC, or colloquially, a “double egg.” And it is entirely real.

In another video, New Scientist spoke with Douglas Russell, the senior curator of birds (and therefore, the egg collection) at the London Natural History museum. He explains:

What generally happens is that as the egg is developing and being pushed down the oviduct, a series of abnormal contractions can occur which can push a developing egg back up the oviduct. What happens is that one egg will be surrounded by another egg. [. . .] Two complete eggs with yolks and albumens will be created.

As Russell notes, the double egg is extremely rare — everywhere, it seems, except on YouTube. Here are three more examples.

Bonus fact : As reported elsewhere by the BBC, in 2006, a group of biologists from the University of Manchester discovered a duck egg which seemed to have something odd moving around inside it. When they cracked it open, they found three live minnows — small freshwater fish — inside. To this day, there is no explanation as to how this occurred.

From the Archives: Inedible Eggs: The trick to hard boiling an egg. A normal, one-egg egg, that is.

Related: A “Hide and Squeak” egg toy set. As an owner of this, I can attest that children love it, but adults, not so much — the pieces get lost very easily. (But the children really do love it.)