How do you make those spoiler things?

[✩Comet✩]

So ive seen a few people use something that says "Spoiler" and then when you click on it, some stuff appears.

Can someone teach me how to do that? I'd like to put some of my art in my bio but i dont want it to take up the whole screen.

[Citrine]

It's [spoiler]Stuff here[/spoiler]

It doesn't show up when you preview a post, so don't let that concern you haha.

[✩Comet✩]

ohhh
thank you so much!!! :,o