When we focus intensely on one thing, we are likely to completely ignore other things not relevant to the thing we are focussing on
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The most famous example was when psychologiests Chabris and Simons created a video of 2 groups of people passing a basketball to each other. The test asked participants to count the number of times the ball was passed to players in a particular color. Half way throu the study, a gorilla walks into the scene, beats her chest and walks out. About half the participants do not see the gorilla.