This is made to allow someone to drain the last dregs of one bottle into a new bottle without having to balance two bottles together then have them blow or fall over and it's a good way to show off to others 'look! This is what a 3d printer is useful for!'

It's made for the standard 27mm OD(of the thread), 20mm ID tops used on many shampoo, hand-wash, cleaning and similar products. It doesn't use threads, just a simple push fit.

Something I made quite quickly for my housemate. It's also adjusted for my sightly over-extruding printer so fit may be a little looser on another.

(If someone knows what those threads are commonly called/sized, leave a comment and I'll update the desc)