All 16 words, sorted alphabetically. Group them into four categories of four.
The four category names, easiest to hardest. No words revealed yet.
Theme name plus one word from each category.
FRUIT, PRODUCT, RETURN, and YIELD all mean the result or outcome of something — as in 'bear fruit,' 'the product of labor,' 'return on investment,' or 'yield a result.'
DAD, INSIDE, KNOCK-KNOCK, and PRACTICAL are all types of jokes — dad jokes, inside jokes, knock-knock jokes, and practical jokes.
CASPER, GHOST, LET ME IN, and TWILIGHT are all movies featuring a romance with a supernatural being — a friendly ghost, a ghost, a vampire, and vampires respectively.
MAZE (a-MAIZE, corn), MUFFIN (corn muffin), SILK (corn silk), and SYRUP (corn syrup) are all things connected to corn — note MAZE is a homophone of MAIZE.