This cute mechanical engineer mentioned in a message that he had designed some manifolds, so that's why I'm asking this (in addition to my general interest in physics, of course). I'm not sure if they were intake or exhaust manifolds, but that's not the problem now. I figured that http://en.wikipedia.org/wiki/Manifold_(automotive_engineering)" was a good place to start, and it says I want to figure out why that would happen. I'd really like to see if I can figure it out on my own with a little bit of guidance. It says that exhaust manifolds are usually made of cast iron. Do I need to look up what types of forces and bonds are at work in cast iron's solid structure? I'm guessing that the liquid water is cooler than the iron, so it will cool the iron where it drips. I can't think of a reason why this alone would cause the iron to crack. Is this enough? Does some chemical reaction take place first, and then it cracks due to some mechanical forces, perhaps because the new chemical structure is weaker? I'm basically just looking for a little direction since I'm curious but don't have tons of time to try every path.