When a sperm fuses with the egg, the egg doesn't wait around. Within seconds, it triggers the cortical reaction, thousands of vesicles just under the egg's surface explode outward, dumping enzymes that harden the zona pellucida, the thick outer shell. That shell physically locks out every other sperm. One egg, one sperm, 46 chromosomes.
But what if that lock fails? What if two, three, even more sperm slip through before the door shuts? That's polyspermy, and it's almost always fatal. The egg ends up with 69 or more chromosomes instead of 46. Cell division goes haywire, chromosomes can't pair properly, the spindle apparatus breaks down, and the embryo either arrests immediately or collapses within days. Nature's filter is brutal, but it's also why most of us exist at all.