Physics of Bowling: The Near Impossibility of a Spare on the 7-10 Split

The 7-10 split is when the only pins standing after a bowler's first shot of a frame are the leftmost and rightmost pins in the back row. It's obviously extremely difficult to knock out both pins on one's second shot, but until I saw this video from Wired, I didn't know how difficult. Professional bowlers, in fact, topple both pins for a spare less than 1 percent of the time! And the only reason a 7-10 spare is possible at all is the chance of a ricochet of the ball or a pin off the rubber doggie-flap-door-like curtain behind the pins, back into the other pin. Without the curtain, there would never be a successful conversion of the 7-10 spare. I highly recommend the above-linked video, which includes interviews with a pro bowler and an engineer, diagrams of a bowling lane and pins, and even a bowling robot!