What jokes, no matter how sad, can make you break the work in one second?

On a train, there are two pairs of mother and son, one is black and the other is white. At noon, the white mother said that she would breast-feed the child and the child would not eat it. When the black mother saw it, she remembered that she should also breastfeed her children. When the white child saw the black child drinking milk, he told his mother that he wanted to drink milk, too, and her mother was about to feed him. He pointed to his black mother and said to her, "I don't want to drink your milk." I also want to drink chocolate milk. "