This coupler is used to connect two 2 liter bottles for demonstrating a vortex and centripetal force.  It has a perfectly snug fit.  There are several of these coupler designs out there, but this one is superior to all of them.  Want a quick and easy way to score an A+ for your kid's science project?  Look no further!  We printed a whole plate of these and every kid in class was able to go home with one after the presentation.  ***This can also be used for the bottle coupler for DIY carpenter bee traps.***