So it's still a tad early to expose W to the mathematical explication of Newton's Laws of Motion, but that doesn't mean we can't start building an intuition for them. Now, kids who engage in non-schooled, extraneous, and wasteful activities such as "play" probably build a pretty good intuition on their own. Perhaps any lesson I design merely risks muddying the mental waters, but in any case, I was not going to pass up an opportunity to heave rocks out of a window.
I asked W whether a light or heavy rock would fall faster, and he thought the heavy rock. This is a great answer and suggests an intuition that Force is directly proportional to Acceleration. I suspect this intuition is largely built from physical play, where one learns something like "the harder you push, the faster you go." When you hold up a heavy object, you feel it push down harder, and as a result you would think it should accelerate faster when released.
But of course, the intuition fails. W learned that a big rock and little rock will hit the ground at the same time. The same goes for sticks, plastic balls, and rubber boots.
So what happened? I explained to W thusly: "gravity acts on all things the same". Sometimes a bigger Force means a bigger Acceleration, but not always. In Newton's famous equation F=mA, a bigger force can result if one holds Acceleration constant but increases Mass.
Of course, then I screwed it all up by dropping a rock next to a leaf: "but you also have to think about air sometimes".