One time long ago, when I was running air start engines, a know it all spectator asked me (in all seriousness I might add), why we didn't spin our engines up on dry nitrogen instead of compressed air. After all, he said "Dry nitrogen is cleaner than compressed air, and won't introduce any water vapor into the engine during the start process".

Duuuuh....
I replied that since dry nitrogen is an INERT GAS, just how in the hell do you intend to start a fire in the absence of oxygen?

You could have cut the silence with a knife..........That was his last question....