what type of mean spirited jokes did george use to play on Lennie?

George had once told Lennie to jump in a river knowing that Lennie did not know how to swim. Lennie almost drowned.

'He damn near drowned before we could get him. An' he was so damn nice to me for pullin' him out. Clean forgot I told him to jump in.'

George swore he would stop playing tricks on Lennie after that.