Q: "When traveling in a straight direction, you will always reach the same point on the globe from where you started. How can this happen if the world is flat?"
A: You need to have evidence for this to be true. Also, define "straight." Remember, the northern point on the compass is, under most circumstances (unless near the centre or deep in the ice wall), pointing toward the centre of the Earth. Therefore, if you follow your compass due east or due west, ending up at the same point you started from, you've just gone around the world in a circle.