During intercourse, a male may deposit his sperm in the vaginal cavity of a female. Each ejaculation contains an average of 280 million sperm cells, any less than 20 million indicates there is an issue with the man's fertility. Such a high number of sperm cells per ejaculation is an evolutionary advantage, because not many of the cells will actually reach the egg. And even out of that small number, only one (and in some rare circumstances, two) sperm cells will unite with the egg.
The sperm cells have about 24 to 48 hours to reach the egg before they begin to break down and are incapable of reproduction. At the top of the vaginal cavity is a small ring of muscle called the cervix, which separates the uterus from the vaginal cavity. When women experience cramping before and during menstruation, the feeling is often caused by the cervix flexing and relaxing to release blood and tissue. When a male ejaculates inside a female, the ideal scenario for fertilization involves the sperm being released right up by the cervix. Sperm are suspended in a fluid called semen, which begins to mix and coagulate with the vaginal fluids. This mixing does two things: it allows the sperm to begin swimming a little farther, including up into the cervix and uterus, and it also initiates a coagulation of the semen, which for a short time makes the sperm "stick" in the vaginal cavity. Because the vagina is naturally a little acidic, the coagulated semen is broken down again and will leave the vaginal cavity on its own. Around the time of ovulation, cervical mucous actually changes in its chemical makeup so that it is thinner and more slippery to allow sperm to have an easier swim up into the uterus.
Here's an idea for an experiment that demonstrates how semen and vaginal fluids create a buffer for sperm to travel up the vaginal cavity and into the uterus. It has to do with cohesive and capillary action. First, put a few drops of food coloring into a glass filled halfway with water and give it a stir. Next, gently dampen a paper towel or napkin with plain water. Then, lower the napkin or paper towel into the glass until it just touches the water- you should see the pigment of the food coloring travel up the napkin! The reason it travels so easily is because the napkin is already wet. For the same reason, sperm have a pretty easy time of getting around in the mucosal membranes of the vagina, cervix, and uterus.
Though the cervix is a very tight ring of muscle, it does have a small opening which allows some of the sperm cells into the uterus. Here's the really tricky part: the sperm cells now have to keep swimming upwards until they reach the egg, nestled in the fallopian tube. It's quite a swim and really depends on how strong a sperm cell is. Sperm cells propel themselves with tails that eventually will begin to break down, so the strongest swimmers are more likely to get to the egg. When sperm cells reach the egg, they must burrow through a thick outer layer, aided by enzymes the sperm release. Hundreds of sperm may push against the egg before one cell breaks through and fertilization begins.