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.
POWDER KEG, TIME BOMB, TINDERBOX, and VOLCANO are all things that can explode or erupt, either literally or as metaphors for dangerous, unstable situations.
ANTIFREEZE (ee), BAZAAR (aa), SKIING (ii), and VACUUM (uu) each contain a double vowel — two of the same vowel letters appearing consecutively.
HOME FRIES, POTATO BATTERY, VICHYSSOISE, and VODKA are all things made from potatoes — a breakfast dish, a science experiment, a French cold soup, and a spirit.
BAKING SODA, CLUB SODA, HYDROGEN PEROXIDE, and VINEGAR are all household substances commonly used as DIY stain removers on fabrics and surfaces.