system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does only one sperm cell get to fertilize the egg? How is multi-sperm fertilization prevented and does this prevention ever fail?
gpt
So, there are 2 main ways this prevention of polyspermy happens. 

1st, there is a fast reaction by the egg upon sperm entry. It depolarizes the membrane, which prevents other sperm from being able to bind to it. Kinda like a magnet.

2nd, is a slow reaction. It is the hardening of this egg membrane, which is called cortical reaction. This hardening of the membrane makes the egg impenetrable to other sperm.