There are more than 6,500 known species of reptiles found on every continent except Antarctica.  Reptiles have scaly skin and are cold blooded (ectotherms), so they must find a warm spot to heat up and become active, and they must find shade to cool off.  In cold conditions they become sluggish and don't move around much. There are four main groups of reptiles: turtles and tortoises; lizards and snakes; crocodiles and alligators; and the tuatara.   The heaviest reptile is the saltwater crocodile, weighing up to 2,000 pounds. The smallest is the dwarf gecko, measuring only 3/4 of an inch.