An old college friend once had a professor who was building a cement boat. We all laughed, because, of course, cement doesn’t float. Right? I mean, this wasn’t a science professor with a formula for helium-filled rock; this was a hippie English teacher. The sly prof got a bunch of us students to come to his property on the banks of the Chesapeake one weekend and, sure enough, in the back yard was a huge gray hull, half-covered with cement. The kids spent the day mixing up sand, stone, and water while the old folks slathered it on. And all the while, I’m thinking, This is nuts. Cement doesn’t float. Later, I realized that steel doesn’t float, either, but tell that to the 5th Fleet. I have no idea what happened to that craft, but I’m hoping it fared better than the entrants into today’s Cardboard Boat Regatta might. I have seen cardboard float…but not for very long. The regatta begins at 2 p.m. at Lake Accotink Park, 7500 Accotink Park Road, Springfield. $4. (703) 569-0285. (Dave Nuttycombe)