Signal non-understanding

An utterance that signals that the speaker has not understood the previous utterance, i.e., did not hear it or could not make sense of it. Instances for that move are “I don’t understand” and variants like “What did you say?”.

Example (1): The following statements signals non-understanding:
  -I don't understand your question
  -What do you mean by that?
  -What do you mean?

Example (2): Below is a part of an explanation dialogue between an explainer and an explainee, where the Signal non-understanding move has been highlighted.
  Explainer:	['Well, it has to do with, a lot with gravity,', 'do you know what gravity is?']
  Explainee:	['No, not at all.']
  Explainer:	["It's what keeps us on the earth."]
  Explainee:	[What?] ---> Signal non-understanding
  Explainer:	["The reason we're not just flying off the earth is", 'because earth has gravity, so if we throw something up,', "it comes back down, so that's why", "when we're walking on the earth,", "we don't fly off the earth because the earth has gravity,", 'and it keeps us down.']