Mind-blowing Trick

DO NOT SCROLL DOWN.
Okay I want you to think of a shape...any shape e.g. a square. Don't think of a square though.
Now think of another shape e.g. square but NOT a square.
Put the first shape inside the second shape.
Now you have to concentrate on your shape with all your will-power.
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You thought of a circle and a triangle, didn't you?