And they all nod their heads up and down for yes and side-to-side for no. Purely random chance. Purely.
Not all humans do this, even if it's common in the majority of countries. Bulgarians do a short nod up for "no", and a sort of head wobble for yes.
In many parts of the Indian subcontinent a side-to-side head "bobble" is a sort of indeterminate "yes/no/maybe".