It is quite natural for young children to associate ‘making bigger’ with adding. It is important for the teacher to value the pupil's generalisation and then at the same time reinforce the mathematics skill of testing out an hypothesis to see if it works for all circumstances.

When pupils add positive numbers together they may notice that 'adding two numbers together gives a larger solution'.  It may be tempting to reinforce this idea.  Instead it may be more appropriate to value and celebrate the fact that the pupil has made a mathematical generalisation and that that is a good thing and at the same time invite the class to 'test this generalisation out for different types of numbers.  This will then reinforce the process of 'doing mathematics' and also reinforce the importance of testing hypotheses to see if they work.  At this stage it may also be useful to link the 'adding makes things bigger' statement with the 'multiplying makes things bigger' statement.  Does this work for fractions, negative numbers, decimals etc.  It is important that any quick summaries or metaphors enhance generalisation rather than impede it.

What happens for subtraction and division?

Pupils might also like to explore different statements about addition and/or any of the operations which are more acceptable.  For example - a better metaphor is "adding means moving a given distance up or down the number-line".