Deceptively difficult puzzle. Not so difficult to make. Two ways to make this are presented:
1) marbles and epoxy. 2) golf balls and a drill press.

Solution video:
   • Four-piece cannonball puzzle solution  

Let's play a simple math game. Wait, don't go away yet! This is interesting. Look at that simple equation at 1:55. Pretend that n gets bigger and bigger, and it eventually approaches infinity. We're just adding marbles, and making it bigger. If n were say, like 200 quadrillion bazillion million gajillion, then the marbles would be so small compared to the overall shape of the puzzle that it would look like the edges had become straight lines. So, it is safe to say that as x approaches infinity, the puzzle approaches the shape of a (perfect) tetrahedron.

This is kinda like our reality; the straightest edge, the roundest curve, the sharpest line- these are all limited in their detail by the atoms that make them up. There is no place that a glass ends and the juice that it contains begins. But mathematics has no such limitation, because we can pretend that there are marbles that are even smaller than the atom. Such is the case with an irrational number, such as pi. After only a few digits, pi is already describing the distance around a circle that is more accurate than the actual atoms themselves. In this case, the atoms are the bumpy marbles. But we want to know the distance around a "smooth" circle. So, let's make the marbles smaller ...and smaller ...and smaller. This is why pi is an irrational number; no matter how small we make the marbles, there are still marbles, so there is still a bumpy circle. Each time we make the marbles smaller, we end up with more digits to pi, and since we can do this forever (in theory), it never stops.

Irrational = ir + rational. Ir = not. Rational = able to be expressed as a fraction (ratio). There is no simple fraction that can describe pi, and π/1 doesn't count, cheater.

I do intend to make a video on exactly this concept, and I have a great idea for expressing it, but it is unfortunate that time is not as limitless. Anyhow, I hope you found this idea to be insightful, so thanks for reading ;)