Lulu was a prostitute, but she didn't want her grandma to know.
One day, the police raided a whole group of prostitutes at a sex party in a hotel, and Lulu was among them.

The police took them outside and had all the girls line up along the driveway when suddenly, Lulu's grandma came by and saw her granddaughter.

Grandma asked, "Why are you standing in line here, dear?"
Not willing to let her grandmother know the truth, Lulu told her grandmother that the policemen were there passing out free oranges and she was just lining up for some.

"Why, that's awfully nice of them. I think I'll get some for myself," and she proceeded to the back of the line.
A policeman was going down the line asking for information from all of the hookers.

When he got to Grandma, he was bewildered and exclaimed, "Wow, still going at it at your age? How do you do it?"
Grandma replied, "Oh, it's easy, dear. I just take my dentures out, rip the skin back and suck them dry."

The policeman fainted