about a subject. For example, the tag "orange" may refer to the fruit or the color, and items related to a version of the Linux kernel may be tagged "Linux", "kernel", "Penguin", "software", or a variety of other terms. Users can also choose tags that are different inflections of words (such as singular and plural), which can contribute to navigation difficulties if the system does not include stemming of tags when searching or browsing. Larger-scale folksonomies address some of the problems of tagging, in that users of tagging systems tend to notice the current use of "tag terms" within these systems, and thus use existing tags in order to easily form connections to related items. In this way, folksonomies may collectively develop a partial set of tagging conventions. Complex system dynamics Despite the apparent lack of control, research has shown that a simple form of shared vocabulary emerges in social bookmarking systems. Collaborative tagging exhibits a form of complex systems dynamics (or self-organizing dynamics). Thus, even if no central controlled vocabulary constrains the actions of individual users, the distribution of tags converges over time to stable power law distributions. Once such stable distributions form, simple folksonomic vocabularies can be extracted by examining the correlations that form between different tags. In addition, research has suggested that it is easier for machine learning algorithms to learn tag semantics when users tag "verbosely"—when they annotate resources with a wealth of freely associated, descriptive keywords. Spamming Tagging systems open to the public are also open to tag spam, in which people apply an excessive number of tags or unrelated tags to an item (such as a YouTube video) in order to attract viewers. This abuse can be mitigated using human or statistical identification of spam items. The number of tags allowed may also be limited to reduce spam. Syntax Some tagging systems provide a single text box to enter tags, so to be able to tokenize the string, a separator must be used. Two popular separators are the space character and the comma. To enable the use of separators in the tags, a system may allow for higher-level separators (such as quotation marks) or escape characters. Systems can avoid the use of separators by allowing only one tag to be added to each input widget at a time, although this makes adding multiple tags more time-consuming. A syntax for use within HTML is to