Naming the ink color of a mismatched color word takes noticeably longer than naming the color of a plain block.

If the word “blue” is printed in red ink and you’re asked to name the ink color rather than read the word, most people take noticeably longer and make more errors than when the word and ink color match, or when there’s no word at all. This delay, first documented by psychologist John Ridley Stroop in 1935, is called the Stroop effect, and it remains one of the most reliably replicated findings in all of cognitive psychology.

The effect happens because reading a familiar word is a highly automatic process that the brain can’t fully suppress, so it interferes with the separate task of naming a color. Because it’s so consistent and easy to demonstrate, the Stroop task is still widely used today, both as a teaching tool and as a real diagnostic measure of attention and cognitive control in clinical and research settings.