Curiosity is an emotion related to natural inquisitive behavior such as exploration, investigation, and learning, evident by observation in human and many animal species. The term can also be used to denote the behavior itself being caused by the emotion of curiosity. As this emotion represents a drive to know new things, curiosity is the fuel of science and all other disciplines of human study.
Although curiosity is an innate capability of many living beings, it cannot be subsumed under category of instinct because it lacks the quality of fixed action pattern; it is rather one of innate basic emotions because it can be expressed in many flexible ways while instinct is always expressed in a fixed way. Curiosity is common to human beings at all ages from infancy to old age, and is easy to observe in many other animal species. These include apes, cats, fish, reptiles, and insects; as well as many others. Many aspects of exploration are shared among all beings, as all known terrestrial beings share similar aspects: limited size and a need to seek out food sources.
Strong curiosity is the main motivation of many scientists. In fact, in its development as wonder or admiration, it is generally curiosity that makes a human being want to become an expert in a field of knowledge. Though humans are sometimes considered particularly curious, they sometimes seem to miss the obvious when compared to other animals. What seems to happen is that human curiosity about curiosity itself (i.e. meta-curiosity or meta-interest), combined with the ability to think in an abstract way, lead to mimesis, fantasy and imagination - eventually leading to an especially human way of thinking ("human reason"), which is abstract and self aware, or conscious. Some people have the feeling of curiosity to know what is after death.
The degree to which a person says that they have curiosity about trivia questions links to activity in both in the Broca's area in their left inferior frontal gyrus, and the putamen in their basal ganglia. This suggests people that are curiosity both activate parts of their brain that comprehend and anticipates information, and those in which such information acts as a secondary reinforcer or reward. Curiosity also increased activity in memory areas such as the hippocampus when subjects guessed trivia questions incorrectly and this suggests that it might act to enhance a person's long term memory for surprising new information. Such activation linked to curiosity predicted better recall of surprising answers one or two weeks later. Dopamine receptors in part of the hippocampus called the dentate gyrus contribute to the generation of curiosity in mice. These receptors are also important for plasticity and learning and therefore are proposed to represent a molecular link between intelligence and curiosity.
A morbid curiosity is an example of addictive curiosity the object of which is death, violence, or any other event that may hurt you physically or emotionally (see also: snuff film), the addictive emotion being explainable by meta-emotions exercising pressure on the spontaneous curiosity itself. According to Aristotle, in his Poetics we even "enjoy contemplating the most precise images of things whose sight is painful to us." (This aspect of our nature is often referred to as the 'Car Crash Syndrome' or 'Trainwreck Syndrome', derived from the notorious supposed inability of passersby to ignore such accidents.)
Curiosity--the spirit of investigation, the urge of discovery, the drive of exploration--is a part of the inborn and divine endowment of evolutionary space creatures. These natural impulses were not given you merely to be frustrated and repressed. True, these ambitious urges must frequently be restrained during your short life on earth, disappointment must be often experienced, but they are to be fully realized and gloriously gratified during the long ages to come. 
- Kang MJ, Hsu M, Krajbich IM, Loewenstein G, McClure SM, Wang JT, Camerer CF. (2009).The wick in the candle of learning: epistemic curiosity activates reward circuitry and enhances memory.Psychol Sci. 20(8):963-73. PMID 19619181
- Saab BJ, Georgiou J, Nath A, Lee FJ, Wang M, Michalon A, Liu F, Mansuy IM, Roder JC. (2009). "NCS-1 in the dentate gyrus promotes exploration, synaptic plasticity, and rapid acquisition of spatial memory.". Neuron 63 (5): 643-56. doi:10.1016/j.neuron.2009.08.014. PMID 19755107.