Well, I collected my first "fart egg" also called a wind egg, witch egg, fairy egg or cock's egg.
They are not uncommon and happen when a tiny piece of tissue gets into the chicken's oviduct and their body mistakes it for a yolk and builds an egg around it.
It's about the size of a bantam's egg, the shell is usually harder and very often is yolkless.
This one was green so one of my easter eggers laid it.
Strange little happenings in chickendom!