Biologists at the University of Manchester were on a field trip to the French Alps when they found a duck egg in a small pond. Noticing its movement, they cracked it open, to discover three live minnows inside
The scientists, despite their combined knowledge of the natural world, are at a loss as to how the fish got inside the visibly uncracked egg.
Dr Matthew Cobb, a lecturer in animal behaviour at the university, said: "As 21st century scientists rather than 17th century antiquarians we think it's unlikely this represents a hitherto unknown mode of fish reproduction."
Dr Cobb and his colleague have written to the magazine New Scientist to see if any reader can come up with the solution.
(Thanks, Iridium and Yashca.)