Advice on Eating Snow

Last night, we received some badly needed precipitation in the form of snow. In my area, it was just a ground covering. The streets were just wet. Up north and west of the city, they received up to 6-8 inches.
The boys were just amazed at the white fluffy stuff as we were getting in the truck to drop the youngest one off at the babysitter and on to school. They were a slightly disappointed that they did not get an opportunity to play in it like they did in the dusting back just before Christmas.
As we were backing out of the driveway, I gave them the old advice that every dad should give his sons: don’t eat yellow snow.
The five year old looked at me in puzzlement and half-guarded belief. He asked, “Why?” Smart boy. Don’t take any advice from anybody without an explanation.
I replied, “Well, in case the dog peed in the snow.”
Still a look of puzzlement. I could see the gears turning in his head. Dog-snow-pee? I don’t quite see the connection yet.
“What color is pee?” I asked.
The three year old got it before his older brother, “Ewww! Grossssss!”
You can imagine what the conversation was like after that.