Fire and Ice
Fire is painful. It's hard to capture, and once you capture it, there are only certain ways you can contain it. However, you can't tame it, not in any way. It will bite you and burn you and fight back, not allowing you to tame it in any way, nor pet it. It's wild, even if it seems calm and gently in the beginning. If you capture it and don't let it go soon, it will burn itself out.
Ice is cold. It's solid, and difficult to use. You can't break it easily, and you will sooner be in pain before you make any progress with it. However, with enough fire, ice will melt down, becoming softer, yet harder to capture or pin down. And if you melt it too much, it will melt into water, which will evaporate, and then you'll have nothing left of neither fire nor ice.
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[I apologize if it seems as if I am writing something similar to your book. This is solely my work. Please do not plagiarize]