This dissertation focuses on when and how children learn about the meanings of the propositional attitude verbs "know" and "think". "Know" and "think" both express belief. But they differ in their veridicality: "think" is non-veridical and can report a false belief; but "know" can only report true beliefs because it is a veridical verb. Furthermore, the verbs differ in their factivity: uses of "x knows p", but not uses of "x thinks p", typically presuppose the truth of "p", because "know" is factive and "think" is not. How do children figure out these subtle differences between the verbs, given that they are so similar in the grand scheme of word meaning? Assuming that this consists in figuring out which of an existing store of mental state concepts (such as belief) to map to each word, this dissertation highlights the role of children's linguistic experiences, or input, with the verbs in homing in on an adult-like understanding of them. To address the when question, this dissertation uses behavioral experiments to test children's understanding of factivity and show that some children can figure out the contrast by their third birthday, while other children still have not figured it out by 4.5 years of age. This is earlier than was once thought, but means that there is a lot of individual variation in age of acquisition that must be explained. And it means that children do not just get better at the contrast as they get older, which leaves room for us to ask what role linguistic experiences play, if we can explore how these experiences are related to the variation in when children uncover the contrast. In order to address the how question, the dissertation lays out potential routes to uncovering the contrast via observing direct consequences of it or via syntactic and pragmatic bootstrapping approaches which exploit indirect consequences of the contrast. After laying out these potential routes, the dissertation uses corpus analyses to provide arguments for which routes are most likely, given children's actual experiences with the verbs. In particular, trying to track the direct consequences of the contrast will not get the learner very far. But alternative routes that rely on indirect consequences such as the syntactic distributions of the verbs or their discourse functions, provide clear signal about the underlying contrast. Finally, the dissertation discusses the consequences of this picture for the semantic representation of "know" and "think", as well as the linguistic, conceptual, and socio-pragmatic competence that children must bring to the table in order to uncover the contrast.