Children like to do what they see the grownup people do. In this case it's washing of the windows.
Preston has been shown how to wash a window. He enthusiastically tried to do it himself.
These type of activities help the child to focus his attention, because his hands can't do anything unless he wills them to work. One can't "will" one's hands unless the focus is on what you want to do with your hands.
These activities will always help to settle a child, especially after he has been upset.