I stress this, too, when I privately tutor. One interesting problem (albeit not directly relevant problem) that came up was "How would you determine the average

I stress this, too, when I privately tutor. One interesting problem (albeit not directly relevant problem) that came up was "How would you determine the average radius of a hair given the number of hairs on your head?". This actually wasn't obvious to me, but my student figured it out right away! Put your hair in a pony tail, and measure the diameter of the ponytail, figure out that area, and divide by the number of hairs. Brilliant!