I have difficult to understand the following law. Can anyone explain it to me using a simple example that can be understood by an 8th grade student.

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> Let $A\_1, A\_2, \ldots, \text{ and } A\_n$ be mutually exclusive and exhaustive events. Then for any other event $B$
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> $$P(B)=P(A\_1B\_1) + P(A\_2B\_2) + \cdots + P(A\_nB\_n) \enspace. $$
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I assume that $P(A\_1B\_1)$ is the same as $P(A\_1 \cap B\_1)$, right? Also, what are $B\_1$, $B\_2$, $\ldots$, $B\_n$? I read information on law of total probability in Wikipedia. It is too hard for me to understand.