Start a fight or argument, as in If Mary finds out he went to the races without her, that will make the sparks fly. In this idiom, the small particles of a fire called sparks are transferred to an inflammatory situation. [Early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.