Virtual life is the illusion of life using complex programming techniques. Petz is a well known virtual life game that models dogs and cats. These are considered believable by the user due to its use of Artificial personality simulation. Unlike Artificial life all of the behaviours are pre-programmed into the system, although the characters are designed to be flawed in order to give the process realism.
For example, a Petz dog will often fail to catch the stick you throw for him during his first few attempts, and once might even slide into a cartwheel and land on his rear. An emotional response from the user at this point demonstrates the effectiveness of this form of simulation. Conversely, a norn trying to eat a door is not faking stupidity - it really doesn't know it's not a good idea!