You may want to use these to teach older preschoolers what nouns, verbs and adjectives are (I do realize they're only preschoolers, but they really catch on to these things quick!)
For younger kids, "thing" "place" "name" "color" "food" and "feeling" are good words to use for the blanks.
We had a few good laughs after reading these aloud.
Check out these Mad Libs if you need some ideas.
