a. Write two vertical lists of 20 colour names in felt tip pen. For the first list, use the correct colour pen for the colour word you are writing (for example, blue pen for the word blue). Repeat some of the colours but place them in random order.

b. For the second list, use different colour pens for the colour word (for example, use a red pen for the word blue).

c. Now ask a colleague to read out loud the colour that the word is written in (not the word itself). Compare the amount of time it takes your colleague to read out the two lists. The second list probably not only took longer but had more mistakes and more hesitations.

William Stroop, who first conducted this experiment, noted that the task interferes with expected behaviour. It is difficult to stop yourself from reading the word (the result of our early learning), which in this case directly impairs performance.