What makes the new ball swing in one place and not in another? Cricket has grappled with this question for well over a century, and while all that grappling has given us numerous hypotheses, we have no conclusive answers.
Bhuvneshwar Kumar may have puzzled over this question as well, if he watched the Thiruvananthapuram T20I in Bengaluru, where he is undergoing "conditioning-related work" - as per the relevant BCCI press release - at the National Cricket Academy after being rested for the series against South Africa. Having just completed a three-match series against Australia during which the new ball hardly ever swung, he may have felt a complex surge of emotions while watching Deepak Chahar and Arshdeep Singh bend it this way and that at the Greenfield International Stadium.
Bhuvneshwar bowled 42 balls in two matches against Australia and conceded 91 runs, while picking up just one wicket.
In Thiruvananthapuram, Chahar and Arshdeep reduced South Africa to 9 for 5 in the space of 15 balls. Both got the ball to swing both ways, and the pitch gave them further assistance by way of its two-paced nature. The jaffas were getting batters out - Arshdeep, for instance, dismissed a pair of left-handers back-to-back, nicking off Rilee Rossouw with an awayswinger before clean-bowling David Miller with one that swerved wickedly late in the other direction - but so were wide and seemingly cuttable balls that stuck in the surface.
If you were a swing bowler, or a fast bowler of any description, these were conditions made for you.
KL Rahul, who helped steer India to their target of 107 with a painstaking unbeaten 51 off 56 balls, described the conditions as being among the toughest he has ever batted in in a T20 game. "The balls were flying, it was nipping around, it was two-paced," he told the broadcasters after the match. "Everything that can be hard for a batsman, that was the wicket today."
Given how much help there was for swing and seam bowling, how much can India take away from their fast bowlers' displays as they finalise their plans for the T20 World Cup? In particular, where does Arshdeep currently sit in the hierarchy of quicks they have picked for that tournament?
Of the four specialist fast bowlers in India's 15 for the World Cup, Bumrah - if fit - is probably the only one guaranteed to start their tournament opener against Pakistan at the MCG. If they're in their best physical shape and bowling rhythm, Bhuvneshwar and Harshal Patel would probably start alongside him. But neither has been at his best in recent months.
The break Bhuvneshwar is getting during this series is a welcome one ahead of the World Cup, given that he has played more T20Is (24) than any other India player this year. But while that should refresh him, India might still be worried about one facet of his bowling when he returns: his performances at the death (overs 17-20). Seven of the 14 most expensive overs he has bowled in this phase during his T20I career have come this year - four of them this month.
India haven't always had their first-choice fast bowlers operating this year, of course, and the presence of Bumrah and Harshal should reduce Bhuvneshwar's death-bowling workload considerably, freeing him up to bowl most of his overs in his best phase, with the new ball. But Harshal hasn't looked at his best since returning from a rib injury, and his death-overs economy rate for India this year (11.00) has been nearly as high as Bhuvneshwar's (11.37).
This is where Arshdeep has really stood out; he has a death-overs economy rate of 7.38 in T20Is this year, easily the best among all India seamers to have bowled at least four overs in this phase - Hardik Pandya, at 10.25, is a distant second.
Arshdeep's end-overs numbers are spectacular, but not hugely surprising, since he earned his India cap specifically because of his accuracy, control and smarts in that phase. But he has shown he can do more; you will remember that he began his T20I career with a new-ball maiden in Southampton, and over his first 12 T20I games he has done quite well in the powerplay, as a first-six-overs economy rate of 7.50 would suggest. His numbers aren't nearly as spectacular as Bhuvneshwar's in this phase - an economy rate of 5.68 while taking 18 wickets at an average of 15.77 this year - but we're comparing a rookie bowling outside his favoured phase with one of the world's great powerplay bowlers.
And when there is a bit of help about, as there was in Thiruvananthapuram, Arshdeep can be genuinely incisive. He is an unusual sort of left-arm quick, more proficient at swinging the new ball away from the right-hander - genuinely swinging it, and not just slanting it across - rather than into the stumps. This is why he gets so close to the wicket while bowling left-arm over - it minimises the angle across the right-hander, so he can keep lbw in play, just about, while bowling outswing - and also why he loves going round the wicket.
In Thiruvananthapuram, though, he showed he can also bowl the other one - two of his wickets came from balls that swung away from the left-hander, and he constantly kept Aiden Markram guessing which way the ball would go while seldom wavering from a tight initial line.
These were, in short, excellent signs for India. The conditions may have been extremely helpful, and as a consequence, it is hard to say whether Arshdeep's display made any real difference to his chances of starting at the World Cup. But he has done all the right things ever since getting called up, and on Wednesday night he did all the right things once again.