He counted them...

This is Bank House in Peebles where John Buchan would often come as a boy on holiday. The red door was famous, so when the building was refurbished it was retained - although moved round onto the gable end for convenience.

The Eddleston Water (Cuddy Burn) flows at the back. There are two separate flights of steps down to the water from the house, and you can imagine that the young boy would run up and down often.

And when you add together the numbers of steps in the two flights you get........ yes, 39!