Question

why don't all eggs and sperms get fertilised

Answer 1

Eggs are normally fertilized in a woman's body within her Fallopian tubes. This event occurs within the first 24 hours after the eggs have been released from the ovaries and can occur only if there are sperm present around the eggs at this critical time. The lucky few sperms after ejaculation which reach the egg in the Fallopian tube surround it , and begin competing for entrance. The head of each sperm, the acrosome, releases enzymes that begin to break down the outer shell of the egg, called the zona. Once a single sperm has penetrated the egg , its cell membrane changes its electrical characteristics. This electrical signal causes small sacs (called cortical granules) just beneath the egg's cell membrane to dump their contents into the space surrounding the egg. The contents swell, pushing the other sperm far away from the egg in a process called cortical reaction. The cortical reaction ensures that only one sperm fertilizes the egg because it blocks further sperm from entering the egg.