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Are the following statements about number always true, sometimes true or never true?
How do you know?
Can you find examples or counterexamples for each one?
For the 'sometimes' cards can you explain when they are true? Or rewrite them so that they are always true or never true?
When you add two numbers you

If you add 10 and take away 1,

When you add 10 to a number,

When you subtract one number from another number you can change the order and the answer will be the same 
What about these statements about shapes?
If you put two squares together

3D shapes have more than

When you cut a square in half

Four sided shapes are called squares 
Three sided shapes are called triangles


You could cut out each set of cards from this sheet (word, pdf) and arrange them in this grid.
Alternatively, you could use these interactivities to organise your thinking:
These tasks are a great opportunity for learners to use reasoning to decipher mathematical statements. We often make mathematical claims that are only true in certain contexts and it is important for learners to be able to look critically at statements and understand in what situations they apply.
The examples here only refer to the key topics of number and shape, but similar statements could be created for any area of maths.
This problem featured in an NRICH Primary webinar in November 2021.
You may want to start with one statement and have a class discussion about whether it is true. Ask learners to think of some examples to illustrate the statement and decide whether it is always, sometimes or never true. If they decide it is sometimes true, they could think about what conditions make it true.
Groups of learners could be given the set of statement cards to sort into the grid sheet. Taking each card in turn they could decide if it is always, sometimes or never true. Then they could justify their reasoning. If they think
it is always true or never true, they could explain why they think this is. If they think it is sometimes true they could start by coming up with cases for each and trying to generalise. The interactivity could be used in pairs on a tablet or computer alongside, or instead of, printed cards. Once all cards are positioned in a cell of the table, a 'Submit' button appears which enables
learners to get feedback on which statements have been correctly placed.
For learners who have had more experience of reasoning it might be good to ask them to try and write their ideas down in a clear way, perhaps for just one or two of the statements at first. This might be in written form, but could be an audio/video recording.
It would be worth sharing ideas as a class at the end. You could pick up on a statement that has been problematic or where there does not seem to be a consensus and support a whole class discussion.
Can you think of an example when it isn't true?
How do you know that it is always true?
Is it possible to check all examples? Is there another way of knowing?
Learners could be asked to come up with their own statements that are always, sometimes and never true within a topic area. Again they should try to justify their reasons and specify the conditions necessary. You may particularly wish that learners create their own 'never' statement, as in fact none of the statements given here are 'never true'.
If appropriate, pupils could try Always, Sometimes or Never? which is based around odd and even numbers and uses similar ideas. Older children might like to have a go at the similar problems Always, Sometimes or Never? Number and Always, Sometimes or Never? Shape.
When discussing as a class, suggest types of numbers to try or specific shapes to consider. Learners often need to start with concrete examples to develop their understanding of a particular concept before they can before they can develop their reasoning within that area. Using a variety of different manipulatives will help all learners to consolidate their understanding and support their arguments.