C.S. Lewis is basically comparing the supposedly “accident” origins of man and the universe to a jug of milk. When he says “upset” he means knocked over. It would be impossible for spilt milk to land in such a way that the splashes would actually form words and tell the story of origins of the jug. He is questioning the the scientists who are supposedly the authority, but even they admit to really just being “accidents” themselves. Hope that helps!

When a jug of milk falls of a table by accident, you will see a splash of milk on the floor. However, looking at the shape of the milk splash will not tell you how the jug was made, or give you the reason for which the jug fell in the first place. It means that the things that you learn from accidents can't be trusted to be accurate.

Thanks so much