_This article is part of a new series about design principles that can serve both as a refresher for seasoned designers and **reference for newcomers to the industry**. Hopefully, the content covered here isn't too obvious and self-explanatory, but it's always great to have a nice quick refresher every now and again, isn't it? — Ed._
In 1910, psychologist [Max Wertheimer had an insight](http://www.leonardo.info/isast/articles/behrens.html) when he observed a series of lights flashing on and off at a railroad crossing. It was similar to how the lights encircling a movie theater marquee flash on and off.