Source: Primary source for this reflection is Eckhart’s interview by Oprah Winfrey on chapter 4 of Eckhart’s book “A New Earth”. Dimtry’s question starts at 53:10. A similar question was asked by Dionne from Port of Spain, Trinidad at 44:06. Unfortunately, the video is no longer available online.
Eckhart describes this experiment in the following video "Dealing with anger, resistance and pessimism" (7:15)