system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do egg cells “know” not to accept more sperm once fertilized? How do they keep the rest out?

This question is courtesy of my ten year old daughter who meant to search “cornception” on YouTube but got autocorrected to “conception.” The videos that came up were fortunately educational in nature. Thanks for reading this far!
gpt
The cortical reaction! 
To ELI5 it:
First, you need to know that egg cells are surrounded by a thick layer of jelly-like goop (Zona Pellucida) Sperm cells have to burrow through that goop to actually reach the egg itself. The first sperm to make it through the goop to the egg sends its genetic information into the egg cell. This triggers a kind of intruder alarm, which releases chemicals (enzymes) that turn the jelly-goop into a hard, thick shell. Any sperm that was still in the goop is now trapped, and any sperm that is late to the party has found the door closed. This makes sure that only one sperm can actually make it to the egg.



Edit: Because a lot of people are asking about twins and how they come about: 

Identical twins are the result of 1 egg + 1 sperm- very early on in development the ball of cells that will become a baby (these are stem cells, cells that can become any part of the baby or the placenta, they haven’t ‘chosen’ a job yet) splits into two balls of cells, each of which develop into their own baby. Since these babies started from the same fertilized egg, they will be genetically identical. 

Fraternal twins are the results of 2 eggs + 2 sperm, each fertilized egg will develop into its own unique baby. This can happen because Mom ovulated two eggs at the same time (the tendency to do this runs in families!). This also means that you can have fraternal twins with different dads, if Mom had more than one set of sperm around when the eggs were available. Fraternal twins are no different than any other set of siblings (genetically) except that they happened to share a womb. 

Bigger sets of multiples are usually a combination of both processes, or the result of fertility treatments like IVF. For example, triplets may be caused by 2 eggs + 2 sperm then one of them splits (so 2 identical babies, plus a 3rd fraternal triplet), 3 eggs + 3 sperm, or, most rare, 1 egg + 1 sperm that then splits, and splits again to make three identical embryos.