because

Từ điển American Heritage Dictionary 4th

be·cause (bĭ-kôzʹ, -kŭzʹ)conj.
For the reason that; since. [Middle English. See because of.] 
Usage Note: A traditional rule holds that the construction the reason is because is redundant, and should be avoided in favor of the reason is that. The usage is well established, however, and can be justified by analogy to constructions such as His purpose in calling her was so that she would be forewarned of the change in schedule or The last time I saw her was when she was leaving for college. All three constructions are somewhat less than graceful, however.·A favorite rule of schoolteachers (but curiously absent from the tradition of usage commentary) is that a sentence must not begin with because. Sometimes, however, because is perfectly appropriate as the opening word of a sentence, as in the beginning of one of Emily Dickinson's best-known poems: “Because I could not stop for Death—/He kindly stopped for me.” In fact, sentences beginning with because are quite common in written English.·Another rule states that one should not use a clause beginning with because as the subject of a sentence, as in Just because he thinks it a good idea doesn't mean it's a good idea. This construction is perfectly acceptable, but it carries a colloquial flavor and may best be reserved for informal situations. See Usage Note at as¹.