Steve Spangler Science sells these on his website:

stevespanglerscience.com/product/1226

After making one for my sons instead, I decided that others needed this as well. :)

You can see it in action here:

youtube.com/watch?v=dv6vQU94wws&feature=player_embedded

It is the simple colored connector that you see Steve screw onto the 2-liter bottles.

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Required items:

* Two 2-liter bottles
* Two 7/8" garden hose o-rings (hardware store)
* One print of this Thing

While the o-rings aren't "required," I like to use them as it makes for a more water-tight seal.


Instructions:

* Print using 2-3 shells for water tightness.
* Cut support material out of the center hole of the printed part.
* Insert a 7/8" garden hose o-ring on both threaded sides.
* Fill one of the two 2-liters 3/4 full of water.
* Screw this Thing onto the full 2-liter and screw a second 2-liter to the other end.
* Invert and swirl.
* Watch the tornado!

Model notes:

If you use the SCAD file to make alterations to this model, it also requires the "2LscrewPositive.11.stl" file from the "2 Liter bottle threads" thing found at thingiverse.com/thing:10489.

Enjoy!

Postscript:

After publishing this, I found parkolay's "20_Oz_Bottle Adapter" thingiverse.com/thing:6090 that performs the same function. While they are very similar, mine will allow others to edit it via OpenSCAD. Also mine has a reducer in the center to make the tornado last longer while parkolay's does not.