In 1953, Igor Sikorsky, the inventor of the first viable helicopter, performed experiments demonstrating that a spinning sphere did indeed experience lateral deflection (that is, “curve”). Although Sikorsky didn’t publish the data, his collaborators did share a summary of findings—among them, that four-seam pitches curved more than two-seam. By 1959, the University of Notre Dame’s F.N.M. Brown produced gorgeous wind-tunnel images of smoke streamlines showing the way a ball with backspin deflected its wake. And that same year, the National Institute of Standards and Technology engineer Lyman Briggs published his own study, concluding that a baseball could indeed arc up to 17.5 inches on its way to the plate. The spinning ball lowered air pressures on one hemisphere, pulling the ball in that direction.

And so, physicists confirmed that a curveball really does curve. But even so, the batter’s perception is different. At the plate, a pitch appears to “break”—jumping or dropping suddenly, rather than smoothly arcing. The neuroscientist Arthur Shapiro has shown that this optical illusion may be due to the way our visual system processes information.

That’s for baseballs, which are made from rubber or cork wrapped in yarn and leather. What about Wiffle balls?

Wiffle balls wouldn’t be possible without the ubiquity of plastic. In postwar America, lab-synthesized plastics flooded consumer markets once they were no longer needed for wartime duties in mortar fuses, parachutes, soldiers’ service-issued combs, aircraft components, or in the Teflon containers used for the Manhattan Project’s most volatile gases. The first Wiffle-ball prototypes were made by cutting holes into the plastic packaging for Coty perfume. Today’s mass-produced Wiffle balls begin life as polyethylene pellets, melted and injection-molded into hemispheres that are then pressure-sealed together.

The asymmetric flow field caused by the Wiffle-ball holes might yield the same result as does the effect on a spinning baseball: a trajectory that curves or bends in the direction of the resulting pressure force. Still, whether the ball tends to curve toward, or away from, those holes is a matter of some contention, actively debated in Wiffle chat rooms and on the field.

Robert Adair, a Yale physicist and the author of The Physics of Baseball, has speculated that the holes, like the stitching on a baseball, accelerate turbulence on the perforated side of the Wiffle ball. Faster airflow may lower the pressure and cause the ball to move toward the holes. However, the Brooklyn College professor Peter Brancazio has countered that scuffing a Wiffle ball “essentially takes the holes out of the equation.” If the smooth, unperforated side of the ball were sufficiently roughened, it might disturb the air more than the holes, reversing the pressure asymmetry and causing the ball to curve away from the holes.