In a recent conversation with my girlfriend, we got into a conversation about the number of continents. I insisted that there were seven, and she insisted that there were five (six if you included Antarctica).
"Seven!" I said, emphatically, knowing this to be an absolute truth, only to be rebuked with an equally emphatic, "Six!"
Since the usual solution of just escalating the volume of one's voice didn't seem a good way of resolving the dispute, I laid a trap for her. "Okay," I said, "name the continents for me."
"Okay... Europe, Asia, Africa, Australia, Antarctica, and America."
"Ah, but America is actually two continents," I told her. "North and South America."
"No... They are one continent," came the reply.
Hm... This seemed to be the root of the problem. Doing a quick search online, I discovered that, indeed, there are six continents in Spanish, and that I was actually not going crazy in remembering there to be seven continents in English (if one were to include Australia and Antarctica).
This all seems to have stemmed from a cultural definition of "continent" that was then surpassed or justified by plate tectonics. Of course, under the plate tectonics world view of land-masses, Asia and Europe become divided up differently, since the Indian plate and Arabian plate are separate from the Eurasian plate. Still, looking at the world as plates does justify a distinction of North America from South America, but not a justification of a split at Panama from South America (and it does require the inclusion of the eastern part of Siberia, since that is part of the North American Plate).
So, in the end, the continents as they stand today are a social construction, and not a physical description based solely on plate tectonics (although the science of plate tectonics can be used to justify certain delineations). In Spanish, though, there are six continents, and in English there are seven. (As a side note, in Japanese, there are -- apparently -- also only six continents, but in this case, Europe and Asia are part of the Eurasian continent, while North and South America are/remain two separate continents.)
Looking through the entry on "continent" on Wikipedia shows that this controversy is known (by some), and the discussions of how to group landmasses into continents includes several different iterations, meaning that the number of continents can range from only 4 (Antarctica, America, Afro-Euroasia, Australia) to 8 (Antarctica, North America, Central America, South America, Africa, Europe, Asia, Australia).