@Background Pony #A39C  
Great explanation. Thanks.

oh this looks like it’ll be a fun evening

@Tetrominon  
English is my first language.

I liked how Spike actually waited until she took a sip to mess with Rarity, you can see the evil smile before he does.

@Background Pony #5177  
@Kamazeustra  
@Tetrominon
 
“I taught twilight to blow… ahem… gl ass “
 
Note the underline! This is key to understanding this joke. Return your eyes to the comic above and observe the speech line from spike in panel 4 is UNDERLINING the ass in glass. This is not an accidental underline, no, this is done deliberately, as you will note it does not happen on any other speech lines in this comic or previous comics.
 
Underlining text in a comic is used to place extra emphasis on a word or in this case part of a word, often as part of a character’s dialogue. Spike placed extra emphasis on the “ass” in “glass”, which is a common and slightly rude term for butt, in reference to the butt-shaped failure of a glass sculpture twilight made in this previous comic:
 

 
Because twilight’s glass looks like ass.
 

Spike: and then I taught Starlight how to blow… my whistle ;)

Rarity’s going to have some girl time today.

Most excellent.

@thps48  
That is not how the word is used at all. Are you ESL?

@Kamazeustra  
”Dying”. Anyone can recover from dying. :3

@Kamazeustra  
I thought that was the joke at first, but wasn’t (and am not) entirely sure. If it was in fact, then being constrained to exactly four panels ruined it completely.

@Background Pony #5177  
He didn’t die.
 
Spike said he taught Twilight to blow and left it at that. He added glass after a pause when he saw Rarity react and she’s trying to cover.

Ha

Just so I’m understanding this properly:  
”Recent Drama”=Sweetie’s police stallion friend dying.  
“Quaint”=Spike actually did teach Twi to blow glass. Why does Rarity have such an adverse reaction?