Figure 1: remove any two matchsticks to form two triangles.
Questions you could ask children before setting the challenge might include:
* How many trigons (triangles) can you see? (There are five: four small ones and the perimeter triangle that makes up the shape as a whole.)
* What types of triangle can you see? Acute-angled or equilateral?
* Subtract the number of matchsticks that make up the perimeter from a baker's dozen. Is your answer less than the fourth prime number? (13 - 6 = 7. Number 7 is the fourth prime number.)
* Count the matchsticks. How many more would you need to make a score?
* Give your name as a shape name (11 could be "hendecagon" or "undecagon").
* If each matchstick measured 2.5cm, would all the matchsticks measure more than a 30cm ruler if they were laid end to end? (No, because 9 x 2.5cm = 22.5cm.)
* What is the quotient of the number of matchsticks that make up the exterior divided by the interior? (6 V 3 = 2) Figure 2: Remove any four matchsticks to form five triangles.
When the children are ready, set them the challenge:
* Ask them to make the shape.
* Encourage them to plan what to do before trying to solve the problem.
* If they are struggling, provide prompts and clues after five minutes by indicating one of the matchsticks that could be removed.
* Award points for each puzzle solved according to difficulty.
* Ask one child to use an OHP to demonstrate the solution.