If you are trying to stop Australia running away with a Test match, a key tip is to hold any catches that come your way.
Dean Elgar and South Africa will have to remember that for next time after the substitute fielder dropped a sitter of a chance to remove David Warner when he was on 26 on Day 3 of the first Test at Centurion.
It was painful for Elgar, who had only just taken to the field. It was also painful for the Proteas, who then let Warner bully and bludgeon his way to a century.
He was finally dismissed for 115, by which point Australia's advantage was almost 400 runs.
Australia were building toward an unassailable lead late on the third day, with a remarkable batting display from South Africa surely now the only way to salvage the Test.