Why are Europe and Asia considered two separate continents?

I was helping my little cousin learn the continents today by pointing them all out on a globe. I told him that a continent is a huge piece of land that sits by itself, and he pointed at the globe and said, "but these two (Europe and Asia) are together". Obviously he's right, and I've known this since I learned the continents myself, but after that I was having a hard time explaining to him why Europe and Asia are considered two different continents. I was going to tell him it's because of how different the people are (the culture, I mean) in Europe and Asia - but I don't think the Japanese culture is anymore similar to that of Syria than that of France. Does anyone know the reason?