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Questions & Answers about Das Wasser kommt rein!
Why do we use rein here instead of just in?
In German, in is a preposition that needs an object, like in das Boot (into the boat). When you just want to say something is coming "in" or "inside" without naming the place, you use a directional adverb. In colloquial German, that is rein.
Is rein a separate word here or part of the verb?
It is part of the separable verb reinkommen (to come in). In a standard main clause like this one, the conjugated verb (kommt) takes the second position, and the separable prefix (rein) is pushed to the very end of the sentence.
Is rein short for herein or hinein?
In this exact situation, it is short for herein, because the water is coming toward the speaker who is inside. However, in everyday spoken German, people use the short form rein to replace both herein (toward the speaker) and hinein (away from the speaker into something else).