Here’s the whole convo between Stiles and Deaton that leads to Deaton talking about a spark:
Dr. Deaton: This part is for you, Stiles. Only you.
[Shows him a container of black powder]
Stiles: Uh, that sounds like a lot of pressure. Can we maybe find a slightly less pressure - filled task for me?
Dr. Deaton: It’s from the Mountain ash tree, which is believed by many cultures to protect against the supernatural. This office is lined with ashwood, making it difficult for someone like Scott to cause me any trouble.
Stiles: Okay, so then what? I just spread this around the whole building and then either Jackson or whoever’s controlling him can’t cross it?
Dr. Deaton: They’ll be trapped.
Stiles: Doesn’t sound too hard.
Dr. Deaton: Not all there is. Think of it like gunpowder. It’s just powder until a spark ignites it. You need to be that spark, Stiles.
Stiles: You mean, like light myself on fire? I don’t think I’m up for that.
Oh wow, that’s a lot more involved than I thought! This came from quotes on IMDB, right? There’s a link to the scripts, I know nonnie has this whole section somewhere. Nonnie, come through!