A rodent's teeth grow at a rate of about 10 centimeters a year. If the animal doesn't keep constantly gnawing and chewing, its teeth would grow until it is impossible to eat. Rats can't sweat. So the way they get rid of excess heat is through their tail. One can tell if a rat is overheating by feeling its tail.
Rats have poor eyesight, especially the pink-eyed varieties.
They sway from side to side while staring at an object, scanning their field of vision to take in as much as possible.