Whence is that knocking is an example of

The correct answer is indication. Hope this helps. Please mark as Brilliant. (:

I assume this is the story by Robert Zimmerman,

One might think that it is awesome living in space, you weigh nothing and you can practically fly. Yet this is the attitude of someone who has never even experienced zero gravity! As Robert Zimmerman describes, weightlessness can sometimes be downright unpleasant! Down here on Earth, we may not think about it at all, but gravity has an important role in nearly everything. For instance, our sense of balance is directly affected by gravity, and when transitioning suddenly it can be messed up, making you feel sick and want to vomit. I think the most important idea that Robert Zimmerman is trying to bring across is that life in space as an astronaut may seem fun or exciting, but the journey to space and life aboard a space station will be rough and literally life changing.