For this task, you'll need some blocks of different shapes and colours, or you could print off and cut out the shapes on this sheet.
Choose a rule, like 'only have four-sided shapes' or 'only have large shapes'.
Challenge someone else to work out your rule.
They can do this by choosing a shape for you to say either "Yes, that obeys my rule and is in my set" (you then put it over on the left) or "No, this does not obey my rule and so is not in my set" (you then put it over on the right).
How did they decide which shapes to choose?
Did they get quicker at finding out the rule?
What was the smallest number of shapes they needed to try?
Could you make some more shapes to add to the set? What would you make and why?
Tell us about some of the rules you chose and how you decided which shapes to try.