Given such a URI, how can we find out what it identifies? We need some way to answer this question, because otherwise it will be hard to achieve interoperability between independent information systems. We could imagine a service where we can look up a description of the identified resource, similar to today's search engines. But such a single point of failure is against the Web's decentralised nature. Instead, we should use the Web itself—an extremely robust and scalable information publishing system—as a lookup service for resource descriptions. Whenever a URI is mentioned, we can look it up to retrieve a description containing relevant information and links to related data. This is so important that we make it our number one requirement for cool URIs:

1. Be on the Web. Given only a URI, machines and people should be able to retrieve a description about the resource identified by the URI from the Web. Such a look-up mechanism is important to establish shared understanding of what a URI identifies. Machines should get RDF data and humans should get a readable representation, such as HTML. The standard Web transfer protocol, HTTP, should be used.

2. Be unambiguous. There should be no confusion between identifiers for Web documents and identifiers for other resources. URIs are meant to identify only one of them, so one URI can't stand for both a Web document and a real-world object.

We note that our requirements seem to conflict with each other. If we can't use URIs of documents to identify real-world object, then how can we retrieve a representation about real-world objects based on their URI? The challenge is to find a solution that allows us to find the describing documents if we have just the resource's URI, using standard Web technologies.