You ask the spectator to think of a simple shape that is not a square and another different one. You build up drama and tell them they were thinking of a circle and triangle.
Just as the name suggests, this trick is purely psychological. When told to think of any simple shape that isn't a square, 95% of people think of a circle or a triangle because they were, along with the square, the first shapes to be hot-wired into our brains when we were babies. This trick is very good and amazes many people.
"Hello, Mike. May I try something with you? Great! I would like you to think of a simple shape, but not a square because everyone chooses a square. Just the first one that comes into your mind. Okay, I would like you to think of another. Remember, just a simple one... And not a square. Got it? Great! Now, try to pass the shapes to me, mentally... Okay, I think I have got a something. I believe you were thinking of... A circle and a triangle. Was I right? Great!"
Hand gestures also help this trick. For instance when asking your spectator to think of a shape, maybe subtly draw a circle in the air, or a triangle. Although, do not make it too obvious.
Thank you for Reading.
Hope this helped.