Swapping 20's is trading child and adulthood horror stories with someone new encountered in public life as a way of bonding with them and creating the first strains of friendship with them. Derived from "What's your 20", CB lingo for "your location" or "where you're at."
In "female rules", if she wants to be seen as "a nice person", and therefore easy to manipulate later on, the recipient is supposed to reciprocate with similar personal revelations. Often involves confessing scandalously damaging personal information, liberally sprinkled with Dr. Phil-style psychobabble.
1) New female doctor you encountered in grocery store wants to be friends with you, so she confesses to you she sleeps with all her younger male patients, then asks you for your advice when she confesses one of them may be gay and might have given her a disease. Four days after you meet her.
2) New female stranger you ran into at a rock club wants to be friends with you, so her first-ever phone call to you is about how her father molested her as a child. The night of the day you meet her.
3) Distant female acquaintance you've seen once or twice at school wants to step up the next level to friendship, so one night out of the blue you get a phone call from her, find out she got your number from someone who shouldn't have given it, and she wants to talk with you at length about her extensive liposuction surgeries and suicidal feelings. Suddenly. Over wine. RIGHT NOW.
The above are yet continuing example of why I agree with men that most women are pathologically insane.
And I'm a woman.
"Yeah, Kate and I were swapping 20's last night over sangria and the stuff she wanted to talk about got out of hand."
"Say, I hear you and Marilyn swapped 20's last night. Did she talk about her 1979 flirtation with bestiality?"
"One reason women's relationships with men go wrong is they decide they and the guy should swap 20's the day after they just met."