RD:“Hey Twilight! You think you can teach me how to use a sword?”  
TS:“Excuse me?”  
RD:“Yeah! It would be so cool!”  
TS:“I don’t know Rainbow… this kind of training requires a lot of discipline and perseverance. Besides I can’t teach you how to wield a weapon just because you think “it’s cool””  
RD:“Come ON Twilight, I can do it. You think I’d be a Bolt if I couldn’t handle discipline? Think about it! You’ll have the fastest sparring partner in all of Equestria”  
TS:“hmmm, I don’t know Rainbow. It’s true that having another Combat trained Element would be nice, and sparring with one of the fastest living things alive would be interesting, but I still don’t think…”  
RD:“Come OOON Twi, just give me a chance”  
TS:sigh“Fine”  
RD:“YEAH! This is gonna be AWESOME!”
 
And this is how Twilight ended up teaching THE BLADE to Rainbow. (Silly little idea I’ve had for a while)