Choose the correct pair of words to complete the roommate's baffled sticky note.

"There is a glowing green jar of mayonnaise in the fridge. _____ on earth did you buy that _____?"

The correct answer is What / for.

To ask about the purpose or reason for something, we often use the combination of What at the beginning of the question and for at the end. "What... for?" means the same thing as "Why?", but notice that we never say "Why... for?".