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Dr. Shapiro's Puzzle of the Day

Today's Puzzle

Wednesday, April 21

You should see an audio player above. Listen and you'll hear an auditory illusion. (I don't recommend listening for the full 5 minutes as this may cause temporary insanity…)

This illusion is analogous to a well-known visual illusion which you are likely to encounter outside what kind of business?

Hint If that auditory illusion was a little too spooky for you, here's a palate cleanser:

   


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Yesterday's Puzzle + Solution

You've probably seen this kind of sticker on the back of a semi:
HOW'S MY DRIVING?
CALL (xxx) xxx–xxxx

The other day, I saw a variant of this sticker. It said

REPORT ILLEGAL WASHING

What kind of vehicle was it stuck to?

Hint This vehicle was at a construction site.

Solution It was a cement mixer. From my brief research into this matter, the cleaning process involves hydrochloric acid (to dissolve dried cement), and the cement itself is also an environmental hazard, so you don't want it to be drained just any old place.

Just for fun, enjoy the most famous song about cement mixers. (No, really, this has been covered by a lot of people.)

Congratulations to yesterday's solvers Leo S. and Mrs. Gregg. Thanks to everybody who made a guess!

About This Site

Though he now teaches mathematics, Dr. (né Mr.) Shapiro's first job in a K–12 school was as a lunch monitor in Davis, CA. It was there that he originated the Puzzle of the Day, even rewarding correct answers with tickets in denominations like "15 points" (though without a clear idea of how he'd ultimately redeem these). Dr. Shapiro's favorite puzzle from this pre-professional era was "Tell me the location of the beehive on this campus."

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 2020 to March 2021.