If you've ever broken an icicle off of a car, railing, or even roof of a building then you may want to rethink doing it again.
For icicles to form, liquid water needs to be present first; whether that comes as rain or as snow that has melted and then will flow off the exterior of an object such as a roof, branch, or car. Once this liquid water hits the air that is below freezing, it will freeze before hitting the ground. This is why we start to see ice forming in the classic icicle shape.
Each time another round of liquid water fall onto the ice and freezes, the icicle grows larger.
But why shouldn't you eat the ice? It all comes down to how dirty the surface is that the icicle formed on. As the water flows down the roof, car, or deck railing, it collects all the dirt, dust, and even animal droppings that were on that surface first.
So to summarize, it's not just the yellow snow that you need to avoid. Sometimes it's also the ice off your roof that can be just as unsanitary.