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It's hard to do, but it does happen. As long as you're careful you can go years with virtually no damage.

My previous all aluminum razor blade scratched a bit easier. I had scratches/marks in a few weeks, which never happened on my Macbook's.
 
It doesn't really scratch that easily, but if you drop a metal object on it, it will dent quite easily (e.g. your old heavy iPad slips out of your hands and the corner hits the computer, there's a good chance it'll leave a small mark .
 
I've been using my Space Gray laptop with a watch on my left wrist (aka frequently touches the surface where the wrists lay and no scratches yet (hopefully not soon either)
 
From personal experience, it isn't easy to scratch the silver variant. I wouldn't know about space grey.
 
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