How to find a space in a string as in " "?

I'm working on a custom made Chat Gui and I would like it so that the chat doesn't cut through a word when I send it to the next line(, so that it doesn't cover the entire screen like when somebody has something long to say.) This is what I have so far.

a=Instance.new("ScreenGui",game.StarterGui)
a.Name="ChatGui"
Gui={"a1","a2","a3","a4","a5","a6","a7","a8","a9","a10"}
for i=1,#Gui do
    local d=Instance.new("TextLabel",a)
    d.Size=UDim2.new(0,500,0,25)
    d.Position=UDim2.new(0,10,0,(i*20)+150)
    d.Name=Gui[i]
end
for i,v in pairs(a:GetChildren()) do
    v.Text=""
    v.BackgroundTransparency=1
    v.TextXAlignment="Left"
    v.FontSize="Size14"
end
function Chat(plr,chat)
    if game.StarterGui.ChatGui~=nil then
        b=game.StarterGui.ChatGui
        if #chat<40 then
            b.a1.Text=b.a2.Text
            b.a2.Text=b.a3.Text
            b.a3.Text=b.a4.Text
            b.a4.Text=b.a5.Text
            b.a5.Text=b.a6.Text
            b.a6.Text=b.a7.Text
            b.a7.Text=b.a8.Text
            b.a8.Text=b.a9.Text
            b.a9.Text=b.a10.Text
            b.a10.Text=plr..": "..chat
        elseif #chat>40 then
            b.a1.Text=b.a3.Text
            b.a2.Text=b.a4.Text
            b.a3.Text=b.a5.Text
            b.a4.Text=b.a6.Text
            b.a5.Text=b.a7.Text
            b.a6.Text=b.a8.Text
            b.a7.Text=b.a9.Text
            b.a8.Text=b.a10.Text
            b.a9.Text=plr..": "..string.sub(chat,1,40)
            b.a10.Text="(Continued): "..string.sub(chat,41,80)
        end
        for i,v in pairs(game.Players:GetPlayers()) do
            d=b:clone()
            v.PlayerGui.ChatGui:Destroy()
            d.Parent=v.PlayerGui
        end
    end
end
for i,v in pairs(game.Players:GetPlayers()) do
    if v.PlayerGui:FindFirstChild("ChatGui")==nil then
        c=a:clone()
        c.Parent=v.PlayerGui
        v.Chatted:connect(function(msg)
            Chat(v.Name,msg)
        end)
    end
end
game.Players.PlayerAdded:connect(function(nP)
    nP.Chatted:connect(function(msg)
        Chat(nP.Name, msg)
    end)
end)