What does what does '' passing comments'' mean in this context ?
when Hofmann made some passing comments about the desirability of a synthetic substitute for quinine, it was unsurprising that his star pupil was moved to take up the challenge. mean?

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The idea of "passing comments" is that the person said the things quickly or without much thought. The original idea of this phrase is that one person was passing by another person and said the comments quickly, as they were going past. Now the meaning changed so that a person does not have to be in motion to give "passing comments". But the comments are fast and not well-thought out.

The point in this passage is that Homann only mentioned the idea quickly but his student took the idea seriously, he became interested and he began to do research on that subject.