“It's crazy how quickly it caught on!”

You're talking with a friend about a song that was just released over the weekend, but it's already extremely popular on the internet.

It's crazy how quickly it caught on!

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(something) is crazy

One of the meanings of "crazy" is "surprising and hard to believe". When you hear surprising news, like that your 18-year-old cousin is getting married, you can say:

That's crazy!

it's (adjective) ("how" clause)

This phrase is used to talk about an idea, plus your opinion about the idea. The idea is something like:

That song caught on really quickly.

Then your opinion about it is something like:

It's surprising how quickly it caught on.

It's pretty interesting how quickly it caught on.

Some other examples of this:

I was surprised by how down-to-earth he was.

That's really cool how you made it look like they're flying.

(something) caught on

To "catch on" means to become popular. You use this phrase when you're talking about ideas, books, comedians, food, products, and anything that lots of people can like.

The phrase "catch on" refers to the beginning of the situation, when people first start to be interested and think that something is popular. After something "catches on", you just say that "it's popular":

It's super popular now, but it actually didn't catch on until the third season.

The past tense of "catch on" is "caught on".