"Er sieht ein bisschen müde aus."

Translation: He looks a little tired.

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What is the use of aus in this sentence?

[kyky]

The verb is "aussehen". he prefixed particle is seperated from the verb. In grammars it usually says that some prefixes stay at the verb and some don't. But that isn't correct. The thing about a prefix (or affixes in general for that matter) is that it cannot stand alone. Another example: "umfahren" has two meanings: 1. "circuit something": "Ich umfahre den Pfosten." (um- as prefix) - "I circuit the pole." 2. "knock something over (with a vehicle)": "Ich fahre den Pfosten um." (um- as particle) - "I knock the post over.".