We know about graphics objects. They're made up of geometry, which are the polygons that make up the shape of the objects, and they have materials, which are colors and textures that make it look nice and make it look like real world things but if we're going to do Physics, we also need to know about the physical properties, not just the graphical properties of an object. We also need a Physics object, and that consists of a number of things. We've got a rigid body. That's your main physical thing. The physics world is made out of these things called rigid bodies. A rigid body, it's a body which means it's an object, and it's rigid, that means it doesn't move in itself. It doesn't stretch, it doesn't bend. You can make stretching and bending things out of rigid bodies but each individual one doesn't stretch or bend and that rigid body is which what has your basic properties like your mass and velocity. As well as a rigid body, you have a physic material, which represents certain other properties like how bouncy an object is or how much friction, like how strong friction is on that objects. That's also really important and really affects the way the body moves and interacts with other things. It's very important to set that physic material. There's a last thing called the collision geometry. What's that? Well, that's a special set of geometry that's only for handling collisions. Why do we need that? Well, when objects collide, you see two objects moving and they move to each other and they interpenetrate and at that point, you create a force. What happens is, in the real world, when two objects hit each other, they don't move into each other. They just stop, but there's nothing to stop that happening in graphics so you actually have to go and check every single polygon of every object against every single other polygon of every other object to make sure they haven't interpenetrated, one hasn't moved into the other. You then need to do some collisions stuff. Actually, that's really quite slow, you've got a lot of calculations to do. That's why when you've got an object, you want detailed graphical geometry so it looks really great but actually, you want your physics geometry to be a lot simpler so the physics can be really fast. To be honest, you can't really tell the difference between how a box bounces and how something that is a box with a little bit of extra detail bounces. It doesn't make much difference to your perception of the physics of something, so you can simplify your geometry down quite a lot. If your object looks a lot like a box, make it a box. If your object looks a lot like a ball, make it a sphere. I mean, sphere collisions are actually very fast to calculate. Or if it's a little bit more complicated, you can build it out of several different boxes or you can create custom geometry, but custom geometry that is a lot simpler than your graphical geometry. So, really, when you work with physics, all you need to do is create a bunch of rigid bodies with physic material and with collision geometries, and when you do that, all you need to do is say I want you to obey the laws of physics and it will just happen, and it will work and it will look realistic and that's a really fantastic thing about physics engines and why they're a really great resource if you're creating VR environments.