You Asked:
What does the fin under a surfboard do? — Matt, Ankeny, Iowa

Marshall Answered:
Surfboards have not always had fins. People once used their feet for steering. But the addition of one or more fins gives the surfboard a rudder that makes it much easier to turn. Fins also lock the back of the board in the water so it is much less likely to skid sideways across the water. Surfboards can have one, two or three fins, and some even have quad fins now. For more info see How Surfing Works