Drive Reduction Theory
How do you know when it is time to get a glass of water? You know because you get this feeling of being thirsty which motivates you to reduce the thirst by drinking water. This is what happens according to drive reduction theory. According to this theory, some physiological need (need for water) occurs that creates a state of tension (you feel thirsty) which in turn motivates you to reduce the tension or satisfy the need (drink water).