Imagine you have a five-sided pencil with “HB Pencil” written on one of its sides; the other four sides are blank.
If you were to roll this pencil on the table, what is the chance the side with the text on it ends up straight on top?
If you answered 20%, congratulations, you remember your high school probability classes.
But that's not the correct answer in this particular case.
In this particular case, the chances of HB Pencil landing at the top is 0%.
Because the pencil has an odd number of sides, which means gravity will force a flat side to the bottom and a "point" to the top.
"Theory" is best when you can apply it to the real world.
P.S. Here's a tip of the hat to Stacked Marketer for including this brainteaser in their newsletter a few weeks ago, which prompted this post.