The diagrams shows how to measure lengths of 1, 3, 5 and 7.
Can you see how to measure a length of 2?
There are rods for 4 and 6, so it's possible to measure all the lengths from 1 to 7.
Can you convince yourself you cannot measure a length of 8?
Can you find another set of three rods that can be used to measure all the lengths from 1 to 7?
What about 1 to 8?
1 to 9?
1 to ...?
How far can you go? Take a look at Longer and Longer to explore further.