Boys on Swings

From Draw Like da Vinci, p. 43: Perspective is the means for creating a three-dimensional illusion on a flat surface. The basic concept in perspective is that people and objects look larger when nearby and become smaller as they recede in the distance, diminishing until they finally vanish. Someone walking away from you soon becomes the size of the tip of your finger. This simple principle of depth and distance is fundamental to the use of perspective. The application of perspective to art and architecture peaked during the Renaissance when Leonardo and his contemporaries mastered it.

The boy standing on the far swing is smaller and more lightly sketched than the seated boy in the foreground. Keeping the background light and less detailed adds to the sense of distance.