Size and Weight:
One of the last theropods to evolve and prosper before the K/T Extinction 65 million years ago, Troodon was unusually brainy by dinosaur standards: paleontologists think it may even have been as smart as small, primitive mammals like opossums (that may not sound like much of a compliment, but you have to remember that most dinosaurs of the Mesozoic Era, especially the plant-eaters, were about as bright as tree stumps). Troodon doubtless owed its advanced brain to its equally advanced predatory arsenal, which included a fast, bipedal gait, stereo vision, and probably a sharp sense of smell. (See 10 Facts About Troodon and How Was Troodon Discovered?)
A relatively slender theropod closely related to the small, feathered dino-birds of the late Cretaceous period (most notably Saurornithoides), the human-sized Troodon lacked the brawn to match its brain--which may explain why it occasionally resorted to feeding on the eggs of other dinosaurs. As to its own reproductive habits, there's voluminous evidence that Troodon cared for its own hatchlings after birth, a behavior shared by a few known species of hadrosaurs, or duck-billed dinosaurs (the most prominent examples being Maiasaura and Hypacrosaurus).
Troodon has been the subject of an amusing bit of speculation by paleontologist Dale Russell, who wondered if this dinosaur might have evolved advanced intelligence if it had managed to survive the K/T Extinction. Russell even created a model of an eerily human-looking "dinosauroid" derived from the Troodon lineage--sort of a snapshot of what Troodon might have evolved into if it had managed to live to the present day.