Simply telling a student to "make an inference" will not teach her how to make an inference.
Researchers suggest that reading is a process of connecting an external text--what we read-- with an internal one--what we already know. As Beers describes this process, these connections are inferences. There are many types of inferences (recognizing antecedents, word meanings from context clues, grammatical functions of words, intonation, author's bias, etc.), which can roughly be sorted into two categories: text-based inferences and knowledge- based inferences. In order for students to learn how to make an inference, a variety of types of inferencing strategies must be modeled and practiced.
Beers suggests the following activities for both modeling and practice: think aloud inferences as you read to the class; read ahead in the text students will be reading and make notes about the types of inferences you are making, then put the passage on the overhead and explain your inferences to students; show cartoons to the class and think aloud inferences that help you to perceive the cartoon as funny.