Then there is Non-Newtonian Nod Theory, which can be examined by riding both a recumbent trike and a folder (though not usually both at the same time).
Newtonian Nod Theory is: Likelihood of returned nod = ((perceived value of nodder's bike / value of noddee's bike) + (perceived seriousness of nodder's bike / seriousness of noddee's bike) * percentage to which noddee considers themselves a cyclist rather than a guy on a bike)) / 100.
Recumbent trikes take us into Non-Newtonian Nod Theory as most cyclists have difficulty assigning numbers to both the value and seriousness of the machine. Folders pose a similar, though, different problem, as although clearly having a very low value for seriousness, they are often owned by people who also also own a bike with a higher seriousness value but whose numbers are an unknown quantity. The faster calculators will have time to deliver a Provisional Nod, which is a nod whose speed and degree of movement is designed to convey the fact that their nod is subject to revision once they have more information. The slower calculators fail to nod as they have passed you before completing the sums, which can result in them being erroneously assigned to the Non-Nodder category.
This post was written in the time saved by ignoring several helmet threads.
Ben