I found the example used in an anti-multitasking book I read to be both simple and compelling. The idea was to write all letters from a to z and all numbers from 0 to 25. Compare the total time between writing the alphabet first and then the numbers (abcd...; 1234...), vs. mixing the two (that is, a1b2c3d4...). The separate approach wins by a large margin, and is accessible to anyone to try themselves.