In Piaget's preoperational stage (about ages 2–7), children think in symbols but not yet in logical operations. Two hallmarks reveal the limits: they fail conservation, and their thinking is egocentric. Pick a classic task, run the change, and watch the error.
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Piaget · preoperational, about 2–7
Four classic taskstap one to begin
The preoperational mindBetween about 2 and 7, a child can use words and images to stand for things, but can't yet perform reversible mental operations. That gap shows up the moment you change how something looks without changing how much there is.