New Joints - What am I doing wrong?

I wanted to make something like this: Wiki, so I made this code:

local partA = game.Workspace.PartA
local attachmentA = Instance.new("Attachment", partA)
attachmentA.Position = Vector3.new(0, 0, -1)

local partB = game.Workspace.PartB
local attachmentB = Instance.new("Attachment", partB)
attachmentB.Position = Vector3.new(0, 0, 1)

local rodConstraint = Instance.new("RodConstraint", partA)
rodConstraint.Attachment0 = attachmentA
rodConstraint.Attachment1 = attachmentB
rodConstraint.Length = 5

The constraint appears as PartA's child, but PartB doesn't move towards it. I made sure to name the blocks accordingly.