Hypothetical thin tubes that carry very large amounts of vacuum energy. They belong to the class of objects known as topological defects. As the universe expands, its temperature decreases and its microscopic properties, such as the strength with which fundamental particles interact, sometime change abruptly in what are called phase transitions. A well-known example of such a transition is when water freezes into ice when the temperature drops below 0°C (32°F). Strings are predicted to form at these transitions. In this context, the lowest energy state of the universe, which is called the “vacuum” although it is not necessarily empty, can have a nontrivial structure in space, and strings appear as defects in this structure.