The conditioned stimulus precedes an unconditioned stimulus or coincides with it. Natural conditioned stimuli are used in unconditioned reinforcement; examples are the sight and smell of food. Artificial conditioned stimuli are more varied and do not have a direct relationship to the properties of the unconditioned stimulus; they become positive or negative conditioned signals only when a conditioned reflex is being developed. Conditioned stimuli that are indirect signals of the food-related, defensive, and sexual reflexes are of great importance in the adaptive behavior of animals.