Now that we’ve created the balloon, let’s make sure it can be moved across the screen. We do this by giving it a physics body. When simulating physics, nodes without physics bodies are not considered.
In SpriteKit, a physics body can be assigned up to 32 different categories. You use categories to separate nodes from each other. Note that we assign the balloon category to the contact test bit mask. This causes collisions between two nodes to trigger a notification.