Mammals. We've all heard about them. A few of us have probably even seen one. But, what are mammals? Where do they come from? Also known as "milk-suckers" or "milk-lizards", mammals are a highly derived group of synapsid reptile. More specifically, mammals are therapsids of the cyndont variety. The milk-lizards are one of only two major groups of endothermic or warm-blooded reptile, the other major group being the dinosaurs or avian reptiles. Milk-lizards and all other reptiles can be distinguished from their fellow tetrapods or "land-fish" by the fact that they lay their eggs out of the water (when they lay eggs, that is). In contrast to their fellow warm-blooded reptiles, the dinosaurs, the majority of mammals appear to have broken from tradition by abandoning the time-honored oviparity of their ancestors in favor of the far more obscene viviparity, excreting their vulnerable young completely unprotected directly through the birth canal, often into a pouch or incubator of some kind. Like their fellow warm-blooded reptiles, the dinosaurs, mammals are very often coated in a soft, filamentous covering, increasing the likelihood they'll be spat out by predators. The most distinguishing characteristic of mammals is the delicious and nutritious lactose-rich mucus which they use to nourish their young and excrete through pores in their skin that are evolutionarily modified sweat glands.