When a man ejaculates, the sperm-carrying semen fires out through the urethra in a seek-and-conquer mission. It may seem that all these millions of sperm are racing each other to the finish, but the sperm have different roles. Some are deemed the leaders of the pack, trying to be the first to cross the line. Others are designed to assist, specifically by blocking other men's sperm from making it to the finish line. The goal of pregnancy, of course, is for a sperm to find an egg during a precise window of opportunity and fertilize it.