How can I make the script choose one random thing?

So if I have a bunch of objects in a model called spreadable and I want it to choose one out of all these children how would I do that?

local Target = script.Parent.Spreadable

function makefire(parent)
	local fire = Instance.new("Fire")
	fire.Parent = parent
end

makefire(Target)

In the above I just said Spreadable and the script chose a random one but it continuously chose that same random one each time I ran it.

You can use the method GetChildren to return to you all of the descendants of the model in a table. Then you can use the method random in the math library to pick a random index in the table:

local target = script.Parent.Spreadable
local descendants = target:GetChildren()

function makefire(parent)
	local fire = Instance.new("Fire")
	local index = math.random(1, #descendants)
	fire.Parent = descendants[index]
end

makefire(target)

Note that this function will error if #descendants is less than 1. You can use an if statement to avoid this error if necessary.