>I have a great double-wall steel bottle (Hydro Flask), which is much better than a plastic bottle because it keeps water cold all day long and is basically indestructible.
They actually dent quite easily. I have a 40oz Hydro Flask with a couple dents from an incident with a spider a couple years ago. I had my bottle on the counter top while I was eating and suddenly a spider was on it. In an attempt to kill the spider, I knocked over the bottle on to the wooden floor (trying to squish it). It dented the bottle near the top and on the bottom. I looked up some videos and it's definitely not just me. Found one where it looked like a whole family dented theirs and they used a trick with dry ice and a hair dryer to fix them. I don't know if it gets harder to fix the longer you wait. Mine still works okay, so I haven't done anything about it.
I'm pretty sure the outside is aluminum, fyi, unless they make multiple types. The inside part is probably steel.
I love my Hydro Flask and have drank more water because of it. My only complaint is that the straw lid gets a lot of crap building up in the small gaps and it seems impossible to adequately clean. Maybe an ultrasonic cleaner could do it.