“It is Lily-white, But What’s In It”

Many years ago, while walking my son to school on a snowy winter morning, he was saying to me that he loves snow. I told him it looks nice and it’s lily-white, but it is dirty… He said that it is not, and a little argument ensued. We were at it for most of the walk when a lady who was in front of us with her children, coughed very loudly, then, she spat in the snow. My son squeezed my hand and I looked down at him then I said, “Do you see it in the snow?”
“No. It disappeared,”
“That’s why I told you the snow is dirty. Many other dirty things are in the snow.”
“I don’t eat snow, but my friends eat it.”
“You should tell them what you saw.”