Sentience is the ability of any living being to think intelligently. Most characteristics of sentient beings are walking upright on two legs, having the capability of communication, and being able to express emotional feeling. Sentient beings are further classified into similar groups by race. Beings not possessing sentience are considered as part of a specific species; animals, beasts, and creatures. Based on this definition, mechanical beings are often treated as second class and not considered sentient.