1. A method implemented by one or more computing devices, the method comprising: sampling first click-through data from a repository, the first click-through data identifying queries submitted by users to a search engine and specific result items that the users clicked from search results provided by the search engine in response to the queries; sampling structured knowledge data from one or more structured knowledge resources, the structured knowledge data providing semantic distances between various nouns identified in the one or more structured knowledge resources; processing the structured knowledge data to obtain second click-through the data second click-through data representing respective semantic distances between semantically related nouns as corresponding click values; and training a model using the first click-through data and the second click-through data as training data, the model being trained using a machine-learning training process, wherein the model is configured to process input linguistic items and identify output linguistic items that are related to the input linguistic items.