Rusting without water? I was under the assumption that corrosion (rust specifically) required water + oxygen to occur. But the word equation below states that only oxygen is necessary. I know this is true but I do not know the explanation behind it. Could someone help me understand? Apparently water magically appears from somewhere.

A middle to high school level of explanation would be ideal, not too advanced.    

metal + oxygen ->  metal oxide 

metal + oxygen + water -> metal oxide
It seems the rust reaction was not well explained.  The following is the formula for iron rusting with water.
$$\ce{4Fe + 3O2 -&gt; 4Fe^3+_(aq) + 6O^2- _(aq) -&gt; 2Fe2O3}$$ 
Water will dissolve the iron because often carbon dioxide is available in the atmosphere to make the water slightly acidic. With the help of acidity electrons can be transported much easier from from dissolved iron to dissolved oxygen. Where oxygen, iron, and water meet is most often where rust pits start to form. 

Without water the formula is the following:
$$\ce{4Fe + 3O2 -&gt; 2Fe2O3}$$
This reaction is very slow.  More energy is needed to initiate the reaction as in the case with burning steel wool, or another means is needed to transport electrons.  Dirt, grime, residues, or any number of things can transport electrons resulting in rust.