You need one of these before you can do this wonderful demonstration. What's amazing to me is that the entry has 34 comments. I mean, what's there to say about kitchen scales?
Advice on buying a kitchen scale (for the candy weighing demonstration)
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Andrew Gelman's Books
Apparently you didn't read the comments. At least half a dozen were drug references.
This is a very interesting demo, and this phenomenon clearly occurs in the real world (aka, "what I do").
For example, if you are sampling classrooms within a school system, shoppers or stores within a retailer, or similar entities that vary in their measure of size, there is a tendency to pick ones that are "representative". Representative classrooms are smarter. Representative shoppers spend more than average.
Sometimes it's a matter of sampling (systematic sampling being the best SIMPLE technique to avoid this). Sometimes it's a matter of filtering the universe ahead of time.