The human egg locks like Fort Knox after it’s fertilized. Scientists finally find out how

During a single ejaculation, millions of sperm embark on their arduous journey to the egg.

On the way, numerous challenges await. These include an acidic environment in the vagina, the cervix’s mucus which can act as a barrier or filter, the many wrong turns within the uterus, and the final challenge of penetrating the egg itself which is encased in a protective layer called the zona pellucida.

Only a few hundred lucky sperm actually make it close to the egg. But, in the end, there is only one winner.