Put two rods end to end.
Now take one rod which is smaller than the length you have just made.
Find the rod which you must use to make it the same length as the others.
Try this with different combinations.
Do you always find the right rod first time?
Can you find different pairs of rods which equal the first pair you selected?
Is there a pattern?
Can you talk about what you have found?
Can you write about it?