How to make useful mechanical things using waste materials?

Hi everyone, my name is Chitharanjan. I'm trying to do something useful on my holidays. So please if anyone knows the answer for above question reply or mail me.

110100101108 years ago
i like to build static constructions from rails etc i find in construction sites its great for stuff like home on a tree etc (use wood for the parts that touch the tree) for moving mechanics i'd first think of what to make and then how - not the other way