I plopped a marble on the floor and explained Newton's First Law of Motion to W: a marble standing still will keep standing still until you roll it. Then I rolled it and explained it keeps rolling until the floor slows it down to a stop. W was unimpressed by my lecture and insistence that this law is in fact a landmark of scientific achievement and human thought (even if it is obvious to any 6-year-old). So we spent the next hour putting Newton's Law into action in the form of the marble run, which has since replaced Uno as W's favorite object of play.
Here is one of the runs W created: