Solidworks 2016-2017 SE

This is for 3D printing. But the part is too long so I need to split it somewhere and design a screw-y type of connector that can be glued. Except when I try to delete the bottom, it takes the whole thing with it. I know this has been asked before, I just cannot apply those answers to my situation. Can someone give me a more practical explanation on how the Parent/Child relationships happen automatically, how to avoid or undo?