You may have stumbled across videos or articles promoting the use of a toaster oven to flatten glass bottles. Please do not attempt this!  While a toaster oven will melt the bottle to the point that it becomes relatively flat, the method involves leaving the toaster oven on for days at a time and pushes the machine far beyond its intended use.  Doing so is just asking for a major injury or house fire. It is not worth the risk.  Just so you know, the lovely flat bottles you often see for sale at craft shows and shops were formed in a standard kiln, not a toaster.