Ray returns from the doctor and tells his wife that the doctor told him he has only 24 hours to live. Given this prognosis, Ray asks his wife for sex. Naturally, she agrees, and they make love.

About six hours later, he goes to his wife and says, "Honey, you know I now have only 18 hours to live. Could we please do it one more time?" Of course, his wife agrees and they do it again.

Later, as the man gets into bed, he looks at his watch and realizes he now has only 8 hours left. He touches his wife's shoulder and asks, "Honey, please... just one more time before I die?" She says, "Of course, dear." And they make love for the third time. After this session, his wife rolls over and falls asleep.

Ray, however, worried about his impending death, tosses and turns until he's down to 4 more hours. He taps his wife, who rouses. "Honey, I have only 4 more hours. Do you think we could....?"

At this point his wife sits up and says, "Listen Ray, I have to get up in the morning. You don't!"
I can explain it to you but I can't understand it for you.