Knol is a Google project that aims to include user-written articles on a range of topics. The project was led by Udi Manber of Google,announced December 13, 2007, and was opened in beta to the public on July 23, 2008, with a few hundred articles mostly in the health and medical field. Knol has no policies regarding sources or neutrality. Some Knol pages are opinion papers of one or more authors, and others describe products for sale. Some articles are how-to articles or explain product use. Other people can post comments below an article, such as to refute opinions or reject product claims.