Brewing a poison would be craftin IMO, but knowing which ingredients to use for certain effects should be knowledge.
basically the same for demolitions. Aüpplying demolitions to a building should be crafting, but knowing which explosive is the best for a special purpose and doing the math is knowledge, potentially with a stunt...
Simply put: here it boils down to practical or theoretical knowledge.
I would say that taking a stunt implies you have read the textbooks, so to say. Know if a character suddenly wants to do something outrageous with his poisons or explosives, you could easily modify with Scholarship or Lore or whatever you feel is appropriate. The car mechanic doesn't need to take Scholarship 5 to know all the parts in and out? The engineer who draws up, creates and adapts such parts would, on the other hand. (Though also a healthy bit of Craftsmanship IMHO)
A simple example: I could repair a plasma TV, as long as I know which part goes where, which is Craftsmanship. Additionally, I could declare that I know what a part does, as part of my practical knowledge. However, I do not know how that part does what it does, which is the domain of physics and all that mumbo-jumbo, otherwise known as Scholarship. The man who invented the plasma TV, could, in my view, declare that, seeing as how he invented the damn thing, he quite well knows how to put one together, can deduct something about other similar types of TV (LCD), but little to nothing about older or different types of image equipment (CRT or projectors).
I hope that makes some sense
