If you ever celebrated Easter with an egg hunt, you’d expect those colorful plastic eggs to hold some kind of surprise. It could be some quarters, it could be a toy, maybe some candy, but you could never be sure until you popped it open. But if you’re cracking a real chicken egg, you’re pretty confident about what’s inside. That’s when Mother Nature can throw you for a loop.

A man who raises chickens in the United Kingdom got one such shock. Check it out:

Pretty freaky, right? Researchers at the National History Museum in the UK figured out what happened. Sometimes a chicken will experience “unusual contractions,” forcing the formed egg to travel back down the oviduct to the ovary. There, a forming egg surrounds it and the two come out as one, nesting doll-style.

This is different from the “double yolk” phenomenon, which is when a hen is ovulating too quickly and one yolk ends up squeezed in a shell with another. This occurrence is more common among young chickens, whose ovulating cycles are still a bit off.