Let’s start with a student sharpening a pencil as quickly as he can.
Let’s add a second student sharpening a pencil as quickly as he can.
And then, we’ll add one more student sharpening a pencil as quickly as he can.
If all these guys kept going at the same rate they were in the video, how long would it take for them to grind away 100 pencils?
Mr. Shauver - Learner Educator 
If you share this one with your class, I’d recommend including both girls and boys, unless you work at a boys’ school.
Unfortunately, the class I used to take the videos didn’t have any female volunteers.
I appreciate the advice, though. I’ve used this problem a handful of times and haven’t found the gender issue to be an issue, but thanks for mentioning it. I didn’t even consider that.