Think of a simple rule about numbers — 'even', 'bigger than ten', 'ends in three' — and do not say it.
Write three numbers that follow it. Your child then proposes a number and you say only whether it fits. Write every number in a yes column or a no column.
They guess the rule when ready. Then swap over, which is much harder than it looks.
Finding a hidden rule from examples is induction, the reasoning that underlies both science and mathematics. Keeping visible yes and no columns lets the child test a theory deliberately instead of guessing, which is the difference between experimenting and hoping.