"Can be going along quite happily, then bing! It does a..."
Here's why:
* Comma: A comma is used after "happily" to separate the two clauses of the sentence. The first clause describes a state of being, and the second clause describes a sudden action.
* Exclamation point: The exclamation point after "bing!" emphasizes the suddenness and unexpectedness of the action.
* Capital letter: The word "It" should be capitalized because it starts a new sentence.
The sentence is incomplete, so you would need to add the rest of the sentence after "It does a..." to complete the thought.