Gather eight mixed objects and ask your child to sort them into two groups any way they like.
Once sorted, ask what the rule was. Accept whatever they say, even if you had guessed differently.
Then muddle everything up and ask for a different rule using the same objects. That second sort is the hard part and the whole point.
Re-sorting the same objects by a new rule requires letting go of a category that just worked — an early exercise in cognitive flexibility, which is the executive-function skill that predicts classroom adaptability more than memory does.