On your tongue are about 10,000 taste receptors. They are called taste buds, but "taste hairs" would be a more accurate name in that these receptors project like hairs from the walls of the tiny trenches that run between the bumps on your tongue. When you eat, the receptors send signals to the brain, which translates the signals into combinations of sweet, bitter, salty and sour tastes.
Newborn babies have few taste buds but soon after birth more buds begin to grow, and by early childhood they cover the top and some of the bottom of the tongue, as well as areas in the cheeks and throat. Since young children have many more taste buds blooming in their mouths than adults, they frequently find foods to be too bitter or spicy. Adults, on the other hand, often seek out bitter or spicy foods because of a declining number of taste buds. In children and adults, each taste bud lives a matter of days before it is replaced
Bitterness can be detected in a solution as weak as one part per 2 million, sourness one part per 130,000, and saltiness one part per 400. It takes much more sweetness to register a sweet sensation - one part per 200.
However, taste buds can be tricked. After you brush your teeth, the usually sweet taste of orange juice seems bitter because of the chemicals left behind by your toothpaste. Conversely, certain chemicals in artichoke make almost anything you put in your mouth for a few minutes afterward seem sweet.