Help with meh rotation script?

So I tried making a rotation script first I got two parts then I place one on top of the other then named it Part1 and the bottom on Part and made a motor on the front for the Part1 to see what it is facing then I made this script to weld it and every time I clicked anywhere it would face the mouse without the bottom part moving but I was not able to do that I tried using the mouse.p but it did not work pls help meh But I was able to make the bottom part not move when the top part moves all I need is to make the part look at the mouse. the parts needs to be unanchored.

wait(1) 
local mouse = game.Players.Player:GetMouse()
    local a = script.Parent.Part1
    local b = script.Parent.Part
    local weld = Instance.new("Weld")
    weld.Parent = script.Parent
    weld.Part0 = script.Parent.Part
    weld.Part1 = script.Parent.Part1
mouse.Button1Down:connect(function(warn)
    weld.C1 =  CFrame.new(0,-1,0) * CFrame.Angles(-- ;-; What do i put here to make the part look at the mouse-- )
end)

If you can help meh that would be great.