How do I tween a unionated part?

Like SandlineTactical said, sadly there are no ways to tween parts. But there are ways to get around this. You see, roblox has added built in objects/functions for each type of class of objects (e.x. Tweening and GUI, BodyMover's and Parts).

Since we're focusing on physical objects (parts), we'll use a BodyMover to smoothly animate the parts.

--Specificly, we'll use BodyVelocity to control what direction we want to move that object and how fast that movement will be.

Now the only thing that's left is to change the Velocity in a certain axis, in a certain sequence, to simulate an animation.

But let's say your more used to tweening than you are BodyMover's? How would you format a sequence (animation), but with the 4 arguments that are always within a tween?

--Let's break this problem down into a couple parts:

1) Since tweening is a function, we'll create a custom function to run specific lines of code.

2) Each argument in the tween itself will be the following:

--The starting position

--The ending position

--The increment for how much the object will cover

--The how frequent will the increment be aplied (to it's position)

The final product should look like:

Part = workspace.Part
OverLoad = false

function Tween(part,start,end,increment,time)
    if Part:FIndFirstChild(¨BodyVelocity¨) == nil then
        Instance.new(¨BodyVelocity¨,Part).Velocity = Vector3.new(0,0,0)
    end
    Part:WaitForChild(¨BodyVelocity¨)
    OverLoad = true
    wait()
    OverLoad = false
    local Rate = start - end * Vector3.new(time/increment,time/increment,time/increment) --quote me if I´m wrong on this part

    local LoopStatus = true
    Part.BodyVelocity = Rate
    while LoopStatus == true do
        if Part.Position == end then
            Part.BodyVelocity = Vector3.new(0,0,0)
            LoopStatus = false
        end
        if OverLoad == true then
            Part.BodyVelocity = Vector3.new(0,0,0)
            LoopStatus = false
        end
    end
end

Apologies, but > tweening can only be done to GUI's. My only suggestion is to use CFrames or the roblox animation editor.