If you looked at a clock in the mirror and the time on the clock face appeared to read 8:20, what time would it be in reality?

When you look into a mirror, it creates a horizontal, or left-to-right, reversal of the image it's reflecting. For an analog clock face, this means everything is flipped across the vertical line that runs from the 12 to the 6. The left side of the clock becomes the right side, and vice versa. A hand pointing to the 9 on the left would appear to be pointing to the 3 on the right. Similarly, the hour hand pointing just past the 8 would appear to be pointing just before the 4.

While you could try to visualize this complex reflection, there's a simple mathematical shortcut for solving this classic puzzle. The real time and the mirrored time will always add up to a perfect 12:00. To solve for the real time, you simply subtract the mirrored time from 12:00. It's often easier to think of 12:00 as 11:60 for subtraction purposes. In this case, 11 hours and 60 minutes minus 8 hours and 20 minutes leaves you with the correct time of 3 hours and 40 minutes.

This principle of horizontal reflection is the same one that made Leonardo da Vinci's famous "mirror writing" so difficult to read without a mirror. It's important to note that this rule only works for traditional analog clocks with hands. If you were to look at a digital clock in the mirror, the numbers themselves would appear reversed, but they wouldn't point to an entirely different time in the same way.