is made up of many small
s of itself, for example a vacuum cleaner
consists of smaller vacuum cleaner
s, which are of course made up of even smaller ones, ad infinitum
A variation on this theory states that the smaller-thing recursion stops after one level, that is, a vacuum cleaner is made up of smaller vacuum cleaners that actually do the vacuuming.
A compromise between these two forks stipulates that there are indeed many levels of recursion, but after a certain point, it's all turtles. We call this the "Turtle Point." This is clearly the silliest of the three theories, and is thus the dominant one.