When Sean Wilson collected a giant egg laid by his pet hen, Rosie, he was in for a much bigger surprise.
Cracking the six-and-a-quarter ounce (181g) egg over a frying pan in his kitchen - roughly double the size of a regular egg- Mr. Wilson was shocked to see another fully formed egg drop out.
"You're having a laugh," he exclaimed in a video that has suddenly become an online sensation, attracting more than a million hits since it was posted on January 16.
"There's an egg inside the egg!"
Douglas Russell, curator of the British Natural History Museum egg collection, says that double eggs are 'extremely rare,' and almost never occur in eggs purchased at the supermarket.
"Several theories have been proposed for the origin of double eggs, but the most likely suggests that the normal rhythmic muscular action, or peristalsis, that moves a developing egg down the oviduct malfunctions in some way," he told the New Scientist.
"A series of abnormal contractions could force a complete or semi-complete egg back up the oviduct, and should this egg meet another developing egg travelling normally down the oviduct, the latter can engulf the former; more simply, another layer of albumen and shell can form around the original egg."
Whatever the scientific reason, the Wilson family enjoyed an unexpectedly large omelet this week.