Hitting a baseball has been called the most difficult thing to do in sports. It’s even harder when almost nobody has any idea where a pitch is going.

The knuckleball fits that description very well, and it’s why so few pitchers in the history of Major League Baseball have mastered how to throw it. Tom Candiotti, who threw the pitch as a major leaguer, and University of Arizona engineering professor Ricardo Valerdi spoke about the pitch.