system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly happens when heat is applied to a bottle filled with water and oil in order to cut it?

I was watching a program about making rum in a 3rd world country where a man made drinking glasses out of normal glass bottles. In order to do so, he filled the bottle about half way with water and quarter with car oil. Then he placed red hot metal rod in it and the bottle was cleanly cut at the line at the top of the oil level. Not at the line where water and oil met, but at the top edge of oil.

So heres my question - what happens there and whats the purpose of having water and oil in the bottle?
gpt
It's to do with specific heat capacities. The specific heat capacity of water is relatively high, meaning that it takes a lot of energy to change the temperature of a mass of water. The specific heat capacity of oil is much lower, so for the same amount of thermal energy input (the hot rod), the oil will rise to a much higher temperature. The purpose is to keep the heat localised to one particular band on the glass, because the difference in temperature across different parts of the glass is what causes the clean cut.