Do you know the answer??

On almost a daily basis, Tyson has at least one question for me that I am not sure how to answer. I'm sure he's has just reached that age where he has a lot of questions about everything, wait a minute, he's always had a lot of questions. Anyways, some of his questions are just hard to answer!
For example, what to you say to this:

Why are the roads sooo flat, when the earth is sooo round?

Any suggestions would be helpful..........