How Can Unwanted Pregnancy Be Prevented?

i heard recently that unwanted pregnancy can be stopped. No need of taking contraceptives or other dangerous means. I was told that a simple salt and water solution would be enough to prevent conception. To go abt it , after intimacy (without using protection) d girl puts a pinch of salt in a half cup of water, mixes it and then drinks it. that stops d sperm from getting to d eggs. how can this be? If this works, why do we still unwanted pregnancies around the place?pls is it true? I need opinions