No real rules that I know of, except that you are limited to 4 words that continue the current story line in some way.
Hyphenated words count as one word.
Taking the story off on a tangent is not only allowed; it is usually encouraged.
The door was opened
but for short period 
The vampires show up
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and keeps door opened? 
then he said softly
have got pampers ready?
：Are you kidding me?
Once upon a time…
Stephany went far away.