It's almost always...
...A tortilla with meat, cheese and lettuce.
What is a taco? A tortilla with meat, cheese and lettuce.
What is a burrtio? A tortilla with meat, cheese and lettuce.
What is a fajita? A tortilla with meat, cheese and lettuce.
What are nachos? Little tortillas with meat, cheese and lettuce.
But seriously. Mexican food is food from Mexico.
And it's good.
Let's go get some Mexican food! I could really go for a tortilla wrapped around some meat, cheese and lettuce.
From a Tsukihime doujin in which a guy chases around a girl with intent to sex her, eventually cornering her. She can't escape so she turns into a cat, a common frustration to aroused men. She thinks she is saved, but the guy unzips his fly and says 'A CAT IS FINE TOO...'.
One day, there was a picture of a cat. It was not Caturday but this specific cat was so long it became a meme and, though the day was not specifically the day of cats, this cat's length became famous.
Many new lusers speculate that this cat is not lengthy, but it is in fact long.