Run a sharp paring knife around the plum, following the slight indent. Take the plum with both hands and twist in opposite directions.
If it is a clingstone plum, one side might fall apart as you twist, but you should be able to get the two halves apart.
Use your paring knife to cut around the stone. If you get lucky and it’s a freestone variety, the pit will come out on its own and the two halves will easily separate.
