Here is a picture of what is called a Fart Egg or Wind Egg which is on the left. A standard Large Egg is in the middle and a Super Jumbo Egg is on the right. The Fart Egg was laid by a full size Barred Rock hen that had been laying for about 4 months. It is merely an anomaly that happens on occasion the egg making mechanism in a hen detects a small bit of reproductive tissue and treats it like a yolk and wraps it in a shell. The Large Egg in the center was also laid by a Barred Rock hen. The Egg on the right was laid by an Easter Egger who has been laying for about a year. These are generally double yolk or triple yolk eggs. Double yolks happen when ovulation occurs too rapidly, or when one yolk somehow gets "lost" and is joined by the next yolk.