I plan to make a strange humanoid body that has its head inside of it, but I can't figure out how to joint it with the body part that the head is inside. For the structure of the body, it can be thought of as a "torso" block with a "head" that is smaller in size inside of it.
You can use a Weld to join parts. If the parts are already in the correct relative orientations, you can do this:
local weld=Instance.new("Weld",Part0) weld.Part0=Part0 weld.Part1=Part1 weld.C0=Part0.CFrame:inverse()*Part1.CFrame