# My Twitter is melting down over Tears of the Kingdom having a chain wrap around a wheel to lift a door open. Can you explain why senior programmers are calling this “black magic”?

Physics simulation starts off simple when you have simple situations, but each additional factor creates additional layers of complexity that the simulation might not handle all that well. Let me try to give you an example of such.

Let’s go with a basic physics problem - I am standing still and drop a ball I am holding in front of me. What should happen? The ball starts with a velocity of 0, gravity pulls it downward, and it falls over time. Creating a general equation to simulate this is not that difficult - it’s pretty basic physics.

Now let’s add another layer of complexity. I’m still holding the same ball, but now I am standing on a mine car that is moving at a constant velocity along a straight rail. I drop the ball over the edge of the mine car. What should happen? The movement of the ball should take the movement of the mine car into account in addition to gravity, causing it to move differently from the above example. Creating a general equation to simulate this result is much more difficult.

And then we add another layer of complexity. I’m holding the same ball and standing on the same mine cart, but instead of the mine cart moving at a constant velocity, the cart got a big initial push and is now slowing down. In addition, we’re no longer on a straight rail, but we’re actually going in circles along a circular rail. I drop the ball over the side of the mine cart. What should happen? How many different factors must the physics simulation take into account in order to calculate the result correctly?

We can make this even more complex. Instead of dropping the ball, let’s throw the ball. Instead of it being a ball, let’s make it bola - a rope with balls on either end - that we’re twirling over our heads before we release it. And we’ll be doing the release while on an accelerating mine cart on a circular rail.

What the engineers on Tears of the Kingdom did was build a physics simulation system that can elegantly handle multiple layers of complexity. That’s an impressive engineering feat, which is why so many skilled engineers are impressed by it. It’s very similar to the fawning over the rope physics from the Last of Us Part 2 a few years back, and for the same reason - it’s an exhibition of physics simulation elegantly handling multiple layers of complexity.