"There isn't a single windmill owner in Holland who doesn't have a second job, for when there is no wind"

This quote by Johnny Ball is describing the truth that windmills are not a reliable source of earnings for people in Holland. Windmills are dependent on the wind to generate power, and when there is no wind, the windmills are unable to produce any energy. Therefore, the owners of the windmills must have a second job in order to make a living. This quote is stressing the significance of having a reputable source of income, and how windmills alone are inadequate to sustain a living. It is also highlighting the significance of having a backup plan in case of an emergency. This quote is a pointer that despite the fact that windmills are a great source of renewable resource, they are not constantly reputable and it is essential to have a backup strategy.