People tend to call any large rodent a rat and any small rodent a mouse. The names are based on general appearance only. Many so-called rats are genetically different from other rodents their size. The same is true of mice.rnrnThe different species of mice cannot breed with each other, nor can the different species of rats interbreed. Mice and rats cannot breed with each other, and there are no rat/mouse hybrids.rnrnA mouse is not a baby rat. Mice and rats have a common ancestor, but they are many different creatures now. They have genetic differences that make them separate species.rnrnIf you’re able to observe a juvenile rat, you’ll see the physical characteristics of an adult rat, such as the shape of the face. Other then being little, the juvenile rat doesn’t look like an adult mouse.