In chapter 4 of Of Mice and Men, Crooks scares Lennie by cruelly suggesting that George has gone and is never coming back. On hearing this, Lennie is incredibly upset. As a man with the mental age of a child, as someone completely dependent on George, Lennie is incapable of handling the prospect that his friend won't be coming back.
Realizing the danger he could soon be in, Crooks stops playing his cruel little game and tries to calm Lennie down, telling him that he was just supposing that George had left.
Although Crooks's game may indeed have been cruel, there was certainly an underlying purpose to it. It would seem that Crooks wanted Lennie to know what it feels like to be all alone in the world, to have no one alongside you to help you withstand life's many trials and tribulations.
Crooks's game could also be seen as a kind of experiment to see if Lennie could possibly imagine what it's like to be a Black man in a racist world. As the only African American on the ranch, Crooks is about as lonely as it's possible to get. Physically separated from the other men, he's forced to lead an isolated existence in a crummy little room that's notably inferior to the accommodation provided for the white workers on the ranch. By telling Lennie that George isn't coming back, Crooks hopes to make him aware of just what that feels like.