# Lesson 10: Collision Detection

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- Change the spheres into axis-aligned cubes. Make sure to change the angles at which they bounce off of each other.
- Change the octree so that each octree node without any children has a depth of at least 4. In other words, make sure that nodes of depth 1 to 3 all have children.
- Make a version of the program where all the spheres lie in the same plane and move in the x and y dimensions. Use a two-dimensional "quadtree" rather than a three-dimensional "octree".

*Next is "Lesson 11: Putting It All Together".*