The Merriam-Webster Dictionary defines common sense as sound and prudent judgment based on a simple perception of the situation or facts. The Cambridge Dictionary defines the concept as the basic level of practical knowledge and judgment that we all need to help us live in a reasonable and safe way. According to Wikipedia, in artificial intelligence research, common sense knowledge consists of facts that all humans are expected to know (e.g., lemons are sour). A distinction can be made between social common sense and physical common sense apart from many other types of common sense. Spatial common sense can be seen as a form of physical common sense.