How do i add damage to this?

local enabled = true
Player = game.Players.LocalPlayer
while not Player.Character do wait(.03) end
character = Player.Character
mouse = Player:GetMouse()
run = game:GetService("RunService")

function onKeyDown(key)
	key = key:lower()
	if key == "z" then
		game:GetService("Chat"):Chat(Player.Character.Head, "Paper!")
		local b = Instance.new("Part")
		b.BrickColor = BrickColor.new("Neon orange")
		b.Shape = "Ball"
		b.Size = Vector3.new(2, 2, 2)
		b.TopSurface = "Smooth"
		b.BottomSurface = "Smooth"
		b.CanCollide = false
		b.Parent = workspace
        local b2 = Instance.new("Part")
        b2.BrickColor = BrickColor.new("Bright yellow")
		b2.Shape = "Ball"
		b2.Size = Vector3.new(4, 4, 4)
		b2.TopSurface = "Smooth"
		b2.BottomSurface = "Smooth"
		b2.CanCollide = false
		b2.Transparency = 0.7
		b2.Parent = b
		local weld = Instance.new("Weld")
		weld.Part0 = b
		--weld.C0 = b.CFrame:inverse()
		weld.Part1 = Player.Character["Right Arm"]
		weld.Parent = Player.Character["Right Arm"]
		local weld2 = Instance.new("Weld")
		weld2.Part0 = b2
		--weld2.C0 = b2.CFrame:inverse()
		weld2.Part1 = b
		weld2.Parent = b 
		game.Debris:AddItem(weld,0.1)
		 local y = Instance.new("BodyVelocity")
        y.maxForce = Vector3.new(math.huge, math.huge, math.huge)
        y.velocity = Player.Character.Torso.CFrame.lookVector*80
        y.Parent = b
        game.Debris:AddItem(b,6)		
	end
end


mouse.KeyDown:connect(onKeyDown)

I'm trying to make it to where if the ball touches a character they take 20% of damage.