I must admit, units, though not exactly genetic to me, were bred into my very fabric by my theme oriented mother. Matching was not limited to the culinary realm. There was the fireman birthday party complete with fire truck cake, fireman party hats, and a ride on a fire truck, yet I could not find cheap fire trucks for favors. Small wonder when I began teaching, my mind thought in units. But just as having a panic attack over fire truck favors verged on the ridiculous, so some units can verge on the ridiculous.
Some units seem to be connived instead of flowing together naturally.
Here are three red flags to beware of in units. If a unit revolves around American history, The Witch of Blackbird Pond is a natural book to read. If the unit launches off into pond life, for the sake of adding science to the unit, then the entire point of the book has been lost as well as the point of the unit. The Witch of Blackbird Pond is about Puritan history, tolerance and intolerance, obedience and disobedience, love and hate, not about pond life. For a unit to be well constructed, each activity should contribute to the big picture rather than strain at minutia or incidentals.