"Dennis gives us chapter and verse on how to open a combination lock."

In the story Tall in the Saddle the boy is delighted to be in possession of a brand new up-to-date combination lock for his bike. Unfortunately he is not in possession of the old schoolboy trick of unpicking a combination lock, and is devastated when his friend Dennis tells him how easy it is.

The trick is still being pulled - watch this modern video that demonstrates exactly the method used in the story. (Don't try this in the streets.)