However, it has no human emotions such as pity or fear unless learned through human contact and never stops until it fulfills its mission or is destroyed or shut down (although it appears to be able to sense moments of urgency or danger, as it is depicted as raising its voice or widening its eyes in shock on several said occasions). Later Terminators are outwardly indistinguishable from humans, but dogs become agitated and bark loudly in their presence; as a result, the human resistance uses the animals to detect Terminators. In the first film, Kyle Reese explains that the T-800 was designed to be an improvement over the earlier T-600 units, which could be easily detected because their skin was made of rubber and not organic tissue. Later models, such as the Guardian from Terminator Genisys or Carl from Terminator: Dark Fate, showed a greater capacity for emotion and physical aging. The most notable science fiction characteristics are that of an expert system featuring strong AI functionality combined with machine learning, and the system can interpret arbitrary non-formalized tasks. The other notable science fiction component is that of a power source which can last 120 years. A trait persistent throughout the series is the faint red glow of the "eyes" when the cyborg is online (or blue in the case of the T-X), which dim to nothing when a Terminator shuts down. In all six movies, the lack of the glow has been used to show when one is out of action. The trait is so characteristic that light-up eyes are often found on Terminator merchandise, with some even replicating the dimming/reillumination effect that occurs during shut down or start up. Construction A description from Kyle Reese from the film The Terminator states that: "The Terminator is an infiltration unit, part man – part machine. Underneath it's a hyperalloy combat chassis, microprocessor-controlled, fully armored, very tough. But outside it's living human tissue. Flesh, skin, hair, blood, grown for the cyborgs." As seen in the movies, a Terminator can withstand standard normal 20th century firearms, crash through walls with little to no damage, and survive explosions to some degree. Repeated shotgun blasts have enough force to knock it down and temporarily disable it, while heavy amounts of automatic fire are able to compromise the organic disguise layer. In Terminator Genisys, a large-caliber anti-materiel rifle proved sufficient to disable a T-800 in