Let's go shopping

Professor : "Now we need only one more thing"
Assistant : "All right, then I'll go and buy one—"
Professor : "Stop!"
Assistant : "What?? But..."
Professor : "I said 'stop', didn't you hear me?"
Assistant : "Oh dear... what do we need, professor?"

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Now, what do they need?

Answer:

Consider what the professor is saying.
I wonder why the professor needs it...
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