In 1935, John Ridley Stroop published a finding that seemed trivially predictable in hindsight: people are slower to name the ink color of a word when the word spells a different color. Seeing the word BLUE slows you down when you're trying to say "red."
The reason is that reading, for fluent adults, is automatic. It happens without intention, and it cannot be suppressed. The moment you see a color word, your brain retrieves its meaning before you've decided to read it. That retrieved meaning conflicts with the ink color task, and resolving the conflict takes time — typically 50 to 150 milliseconds per trial. Knowing about the effect doesn't help. The interference happens upstream of your awareness.
This experiment runs 24 trials. For each word, click the button that matches the ink color — not what the word says. Your reaction times are recorded. At the end, you'll see your own Stroop effect: the gap between how fast you are when the word helps versus when it fights you.
Reading, for fluent adults, is not a voluntary act. It is a reflex. The word's meaning activates before you decide to read it, and that activation doesn't stop because you're doing a different task. The Stroop effect is the measurable cost of that automaticity.
This is one of the few cases where an internal process is directly visible in behavior. You cannot feel the conflict happening. You don't experience the moment your brain retrieves "blue" from the red ink of BLUE. But you slow down, and that slowdown is the trace of something that would otherwise be invisible.
The effect also doesn't go away with practice or knowledge. You know about it now. Run the experiment again — the interference will still be there. Automaticity, once established, cannot be locally suspended. The process that produces the interference is not under the same executive control as the task you're trying to do.
Related: the McGurk effect (entry-456), the visual blind spot (entry-458). In all three cases, the brain does something automatically that cannot be overridden by knowing it's happening.