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.
BONUS, EXTRA, GRAVY, and ICING all describe something additional and welcome beyond what was expected or required — an added perk or benefit.
CONDENSATION, FREEZING, MELTING, and VAPORIZATION are all phase transitions that liquids (like water) undergo as they change between states of matter.
FESTIVUS, REGIFTING, SHRINKAGE, and YADA YADA are all concepts that were coined or popularized by the TV show Seinfeld.
An apostrophe (') can indicate a CONTRACTION (can't), a FOOT (measurement symbol), a POSSESSIVE (John's), or a QUOTE (opening quotation mark in some styles).