Two signs that I have seen in labs where I have worked:
... Pick Any Two
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Answers which Require Thought: $3
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Dumb Looks are Still Free
This last reminds me of when the engineers back at the environmental consulting business I used to work at would yell at me: "I need a number!" "Six!" I would answer immediately. "...Oh, did you want me to run your sample? That will take a while."