The human body can be stimulated by certain things. This stimuli is measured by sensory receptors and the energy is converted to something our brain can understand. There are receptors in our tongue which can measure four main things.
This is the simplest receptor found in the mouth.
Bitter tastes are important, since poisons are usually bitter and therefore we need to know whether or not to spit a bitter food out. This is the most complicated receptor found in the mouth.