I'll show you mine if you show me yours!
Young children looking at each other without any clothes on.
Children tend to be very curious about things, including what the opposite sex looks like under their clothes. So naturally, sometimes they try to satisfy that curiosity by negotiating with someone roughly their own age. It's called "playing doctor" since it's frequently put in the form of a game where one child is the doctor, and the other the patient, who takes off their clothes for examination like at the real doctor's office.
For legal reasons, this game is almost never played all the way in visual media, being mentioned off stage, or interrupted before too many clothes come off. A common "bit" is for a parent to catch the children doing this, and completely overreact making the kids more confused and traumatized than the game ever would have. A common subversion is for precocious or overly intellectual children to actually be roleplaying medical professionals without any intent of taking off clothing. Indeed, this subversion is much more common in media than the "straight" use, as you'll see in the examples section.
It's considered kind of cute for very small children to do this, less so as they get older, and if either participant has hit puberty, they've lost the Children Are Innocent excuse. Indeed, if anyone past puberty suggests "playing doctor," it's an Unusual Euphemism. And if one child participant is significantly older than the other, it's usually a bad sign.
Not to be confused with "playing The Doctor", Playing Dr. Mario, Playing House, Playing "House", or I'm Not a Doctor, but I Play One on TV.
When Megumi from Boku no Futatsu no Tsubasa was little, she tried to play doctor with her cousin Hiromi and ended up giving her a seriously wrong impression. Megumi is a hermaphrodite.
In the series Oniichan No Koto Nanka Zenzen Suki Ja Nai N Dakara Ne, the main male character Takanashi did this a lot with his childhood friend Iroha when they were children, mainly an excuse to see her naked. Because of this, she now regularly stalks him under the belief they are a couple.
Keima winds up on the wrong end of this in The World God Only Knows. During an arc where Keima is undergoing Mental Time Travel to when he was six, a first grader with a Precocious Crush discovers one of her grandfather's porn magazines and wants to copy them with Keima. Keima is fully aware of and Squicked by how just plain wrong this situation is. He successfully turns it around and arranges the situation such that they wind up having a chance to meet the grandfather, whom Keima needed to speak to.
Washu gives this reasoning when Mihoshi walks into her lab and catches Washu in a nurse's outfit and Tenchi stripped to his boxers (and Washu's hand in his boxers) in the seventh episode of Tenchi Muyo!. Mihoshi, being The Ditz, takes it as meaning "the game" and ponders about joining in before causing trouble for the poor scientist.
A very dark take on the concept by a French comic guy: one kid asks the other to play doctor; he replies "Sure, I got a surgeon's costume for Halloween." Next panel has the parents congratulating themselves on getting the kid such an intellectually-stimulating toy (the surgeon outfit), while a large pool of blood leaks out from behind a closed door.
There is one example in YoungSpirou... only it turns out both of them are cheating. Also, the cover for Tome 3, as pictured above.
X-Men has Lillandra misunderstood when she proposes Charles to play doctor. When he yells, she clarifies she's talking about actual medicine.
In Titeuf, the hero is asked by a girl to play doctor, and he proceeds to… simulate an injection.
Subverted in a FoxTrotcomic in which Jason and Eileen simply role-play various medical bureaucrat lines to each other.
Jason: Have a seat and I'll see you in four hours.
A straighter example of "I'll show you mine if you show me yours" is from the movie The Ice Storm, starring a young Christina Ricci. (It's consistently the most popular clip on Hulu.)
Blade Runner. While talking to Rachel about her memories, Deckard reminded her about the time she and her brother played doctor. Her brother showed her his, and when it got to be her turn, she chickened and ran. The memories were actually implanted in her: they came from Tyrell's niece.
Happened in a movie called Anatomy of Hell. You actually saw the genitalia of the little girl playing doctor, Carolina Lopes.
Schlocky horror film Milo has the title character, as a child, show a series of girls his abortionist father's collection of preserved fetuses (the children are all Nightmare Fetishist types) in exchange for one of them playing doctor with him. Unfortunately for the chosen girl, his idea of "doctor" involves scalpels.
In Teeth, the horror-comedy about the girl with the Vagina Dentata, the step-brother has a scar on his finger, and it is revealed that this is from "playing doctor" with his step-sister in the pool as children.
A variation and ultimate subversion occurs in X-Men: First Class, Erik and Charles are recruiting mutants, one of which, happens to be a stripper in a swank lounge. They bring her into the VIP room and say, "We'll show you ours if you show us yours." She takes it the wrong way until she realizes he is talking about mutant powers.
A rather old joke: a young girl mentions to her mother that she and another boy have been playing doctor. When the mom starts to freak out, the daughter says, "Oh don't worry! We've been playing HMO doctor. He didn't even get to see me!"
Two babies, a boy and a girl, are lying next to each other under the covers. The boy asks, "how do you know which one of us is the boy and which one is the girl?" The girl looks under the covers and then back at the boy, and says "I'm the girl and you're the boy". "How do you know?" the boy asks. "Simple," the girl replies, "I'm wearing pink socks and you're wearing blue socks".
Another old joke has a little girl's mother complain to a little boy's father about the children playing doctor. The man tries to dismiss it as childish curiosity, and the mother snaps, "He took out her appendix!"
In the Philip K. Dick novel Eye in the Sky, a little boy is disappointed when he finally convinces the neighbor girl to show him hers — and there's nothing there. Which turns out to be the same thing he's got down there! They're in a Fisher Kingdom run by someone who finds genitalia icky.
This occurs in the novel Rubyfruit Jungle by Rita Mae Brown when the protagonist Molly is young, and she starts charging for people to see her friend Brock's junk.
In one of humorist Patrick McManus's short stories, he recounts the time as a kid that he got invited over by his neighbor Olga Bonemarrow to "play doctor"; it turned out the girl was referring to a mindnumbingly dull medical-themed board game.
Referenced in Buffy the Vampire Slayer, of course. As children, Willow and Xander played doctor in the same way as the FoxTrot example, and Willow doesn't know that there's another way to play it, leading to a bit of confusion when she references those games. When Willow asks Buffy how it's supposed to be played, Buffy looks embarrassed and claims she never played (Cordelia gives her a "Yeah, right!" look).
An episode of Maude, in accordance with its then-edgy subject matter, had parents reacting to finding their kids, a boy and a girl, naked playing doctor.
In an episode of That '70s Show, there's a flashback, where it's revealed that Kelso played doctor with Jackie when they were little. Kelso remembers to this more fondly than their first date, because he "got further".
Subverted on Full House: the kid yelled "don't open the door, Daddy" because she and a male friend were playing eye doctor, and the eye chart was on the back of the door.
On Mash, Hawkeye would regularly use "want to play doctor?" as a pick-up line on the nurses.
Inverted on The Big Bang Theory, when Leonard and Penny come into the apartment to find Sheldon and Amy playing doctor, Star Trek style; both are fully clothed ('natch), Sheldon is lying on the couch and Amy is running a medical tricorder over him.
Sheldon: I'm in hell, Leonard. (turns to Amy) Don't stop!
Brother says he's tellin, bout you playin' doctor with that girl Just don't tell I'll give you, anything you want in this whole wide world
In The Very World of Milton Jones, he takes it extremely seriously, ordering various things of his "nurse" (oh yeah, they're technically playing "doctors and nurses", behind the bike sheds, natch); his final line is:
"... and for GOD's sake put your knickers back on, this is a sterile environment."
Seven:(while waiting for Clover and Junpei) You playing doctor in there? Junpei: Maybe. Jealous?
Subverted in Ozy and Millie as it's used to parody health insurance. They wonder why they're so concerned about playing doctor and conclude it's out of worry it'd make them cynical. Keep in mind that Ozy is normally a Half-Dressed Cartoon Animal anyhow.
Mr. Cotswolds: Mark, where's your sister? Mark: She's upstairs playing Doctor with that Kyle boy. Mr. Cotswolds: Oh, all right. Mr. & Mrs. Cotswolds: What?! (they dash to her room) Mrs. Cotswolds: Rebecca! Aaah! Aaaaah! Noo-hooo! Mr. Cotswolds: Rebecca, don't play that perverted game! (reaches the door and opens it; before them is Kyle on his back on a low table covered by a blanket; Rebecca, in scrubs, stands on a chair cutting away at Kyle's hat; both look at her parents) Rebecca: I have to extirpate a lesion in his cerebral cortex, or risk ih-nfection to the synaptic responses.
They did this in Rugrats, sans the doctor role-play.
A rather surprising example came from King of the Hill between Bobby and Connie. The two of them, after a large misunderstanding about Connie thinking Bobby doesn't like her because she hasn't developed yet, decide to take off their shirts in front of each other and see each other's bodies. While Bobby gets his shirt off, Connie is interrupted by her father entering the room. Being that Kahn doesn't like Bobby already, and hates the idea of Connie being exposed to what he assumes to be rednecks, he's pretty pissed off. This example is very strange because the two of them are clearly at that age when they're starting to notice the opposite sex, and experience sexual urges, and are at least partly playing the game because of those urges, and yet the entire affair carries a clear feeling of innocence. There is the fact that sex wasn't on Bobby's mind at all. He thought couples took their clothes off and looked at each other, nothing more.
In one episode of Family Guy, they flashed back to Peter and Lois as children. When Peter first sees her he says, "Wow, I'd like to play doctor with her... and remove her appendix before it burst, causing sepsis."