In Chapter 5, Holden opens his window and makes a snowball with the intent of throwing it at something. He changes his mind because he takes notice of the pristine snow that has fallen:
"I didn't throw it at anything, though. I started to throw it. At a car that was parked across the street. But I changed my mind. The car looked so nice and white. Then I started to throw it at a hydrant, but that looked too nice and white, too. Finally I didn't throw it at anything." (Pg. 36)
On one level, this shows Holden's respect for nature, which symbolizes the respect he desires from the world around him.
On another level, this scene is a motif that emphasizes the theme of Holden's difficulty in grasping the concept change. Like the ducks on the pond, winter is fleeting. The ducks and the snow prove to Holden that change is necessary--it's inevitable.
The snowball is a symbol. It symbolizes Holden's fear of corrupting innocence or childhood memories. Holden is afraid to mess up the "whiteness" and "niceness" of the snowball as it symbolizes the purity of Holden's memories.
The snow represents Holden's emotions. Throughout the book he is constantly on the verge of directly sharing his true emotions with others, but instead goes with an indirect attention getting approach. The idea of throwing a snowball, and the snowball smashing, signifies his inner emotions being let out. However, he is not able to destroy the nice look of the fire hydrant and the car, just as he is unable to destroy the nice facade of his life (He is a privileged rich kid). Instead, he takes the snowball back inside and packs it in tighter, just as he packs his emotions.