So we've got that round pastry. Case closed.
But there's that number, too. You know, 3.14159... yada yada yada.
Where did that come from?
Actually, the concept of pi has been around since the ancient Chinese and Hebrew civilizations, several thousand years BCE, and was considered to be simply 3. But it was the Greek mathematician Archimedes who started to calculate this more exactly.
You see, one day, this guy Archimedes was sitting around with a circle. He took the measurements of the radius, diameter, circumference, and area -- just for kicks -- and he decided to do some calculations* with them, being the cool little mathematician guy that he was. He noticed that he kept coming up with this weird number. It started with 3, then there was a decimal point, and then a whole freakin' lot of decimal places. Just thinking about calculating it started giving him a headache.
So he thought, "I think I'll call it 'pie,' since after all these calculations, there's this number. Makes sense, doesn't it?" But the thing was, Archimedes from Greece, so he was rather partial to saying things in Greek, with the Greek alphabet. Conveniently enough, the Greeks had (and still have) a letter that's pronounced exactly like the round pastry. "Okay, I'll call it π, since it sounds like like 'pie.' Well, in English it's spelled P-I, not P-I-E, but since English hasn't been invented yet, I'm not too concerned."
π~=3.141592653589793238462643383279502884197169599375105820974944592307816406286208998628034825342117067982148086513282306 647093844609550582231725359408...
* For more on these calculations, look at the next section.