In this paper, we investigate knowledge reasoning within a simple framework called {/it knowledge structure}. We use {/it variable forgetting} as a basic operation for one agent to reason about its own or other agents' knowledge. In our framework, two notions namely agents' {/it observable variables} and the {/it weakest sufficient condition} play important roles in knowledge reasoning. Given a background knowledge base $T$ and a set of observable variables $Oi$ for each agent $i$, we show that the notion of agent $i$ knowing a formula $/varphi$ can be defined as a weakest sufficient condition of $/varphi$ on $Oi$ under $T$. Moreover, we show how to capture the notion of common knowledge by using a generalized notion of weakest sufficient condition. We also discuss possible applications of our framework in some interesting domains such as the automated analysis of the well-known muddy children puzzle and the verification of the revised Needham-Schroeder protocol.