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Impression refers to the mental bonding that occurs between a dragon, fire lizard or wher and a human. When a fire lizard or wher is Impressed, it can give its trainer or owner impressions about what it is feeling, and it has the ability to project its thoughts to a limited extent. Dragons, however, are capable of full-fledged mental communication with their riders.
Fire Lizards, the genetic predecessors of dragons, have the ability to form a mental bond with the first person who feeds it after hatching. This bond is almost a reflexive, instinctive thing, and allows the fire lizard limited communication with its owner, generally appearing as emotions or images. Fire Lizards are not particular in who they Impress to, and a human can Impress more than one fire lizard. Watch-whers are also capable of impressing, but are capable of changing who they are bonded to.
For Pernese dragons, however, Impression is not a choice but a necessity. Impression being necessary for dragons was the deliberate work of geneticist Kitti Ping Yung, who recognized that introducing large, flying carnivores that could think for themselves into the planet's ecosystem, without a definitive control, would be disastrous for both humans and all other species on Pern. A dragon growing up without human guidance, or outliving their rider, could potentially become extremely dangerous. Thus, hatchling dragons retain a strong instinctive desire to Impress to a human. Dragons who do not find a suitable candidate to Impress to are known to go between. Thus, it is traditional for weyrs to offer large numbers of candidates, to ensure that the hatchlings have sufficient choice. However, hatchling will occasionally choose a human who is not a candidate, instead choosing someone in the audience.
Hatchling dragons are born knowing their name, and make their choice known by communicating their name to their desired candidate. Successful Impressions often bring intense feelings of joy to the chosen human. For a dragon and a human, Impression is more than merely the melding of two minds together. They literally become one being with two bodies, their souls intertwined to the extent that there is no way for them to tell where dragon begins and human ends. Thus, it surpasses the familiar concept of telepathy. The emotions, thoughts, desires, wants and needs of both dragon and human partners are known to each other. Though a human can live, although shattered and incomplete, with the loss of his or her dragon, if the human partner dies the dragon will instantly go between as it cannot comprehend or fathom life without its human counterpart. In the case of gold dragons with unhatched eggs, the maternal instinct can prevent the dragon from committing suicide until her eggs have safely hatched and the hatchlings themselves have impressed.
While it appears that there are no limitations in who can impress a Fire Lizard, only certain humans are capable of impressing dragons. Potential candidates have latent mental talents that can be sensed by other dragons. Different colors of dragons will impress different groups of people. While dragons only Impress one person, it has been theorized that human could potentially reimpress following their dragon's death, given that humans can Impress multiple fire lizards. However, in the only tested case, Brekke, who was presented as a candidate following the death of her queen, refused to go through with Impression, leading the hatchling to choose another candidate.