In another mystery solved, the sender of the puzzle was Graham, who got it from a puzzle hunt for math students. There's a complete official solution. Graham, I might have had a better chance if you'd included the puzzle's original title... 😄
Solution
A few of you got as far as I did—you solved the clues, noticed that they form pairs of anagrams (such as CARTELS and SCARLET), and then stared at the grid in confusion. Only Charlie and Yana figured out what to do from there!
Here's how they filled in the grid...
Notice something dang clever...
I thought ENRAGED and GRENADE were one of the anagram pairs, but it looked sloppy for one of the clues to be an outright answer. Well, it's not. Enraged clues ANGERED!
And the final answer is...
TESSERA, obtained by reading the circled/shaded cells in order of the Across clues they belong to (i.e., rainbow order in Charlie and Yana's grid).
Congratulations to yesterday's solvers Charlie and Yana. Thanks to everybody who made a guess!
Ten years later, Dr. Shapiro revived Puzzle of the Day at Proof School, writing each day's puzzle on a name tag. After 600 puzzles or so, he was just starting to feel normal about students reading his chest all the time when campus closed and the puzzle, like the rest of our lives, moved online. New puzzles are posted daily on school days.
Want to catch up on old PotDs? There's an archive currently containing puzzles from March to December 2020.