It's a sight gag you've seen David Letterman, in balmier days, pull off a million times.
He'll be doing some bit on his show with mock danger involved -- some experiment by kid scientists, say, or feeding an animal you wouldn't want crawling around your bedroom.
And then, suddenly, with a blood-curdling shriek, he'll pull his hand back or pretend to hold some body part in acute pain.
He'll mime violent calamity -- not with Johnny Carson's pretend infantilism but like the class clown in the school parking lot; or maybe a jock hoping for a good call on the playing field when the ref is watching.
He hasn't done it all that much lately. And now I think I know why.
I've just read one of the most eye-opening and informative books ever written about stand-up comedy, "I'm Dying Up Here: Heartbreak and High Times in Stand-Up Comedy's Golden Era" by William Knoedelseder (Public Affairs, 280 pages, $24.95).
Let me move a few feet out on a limb and call it one of the books of the year for any student of American television and pop culture.
It's the perfect companion to another great recent book about the metamorphic emergence of stand-up comedy in the '70s, Richard Zoglin's "Comedy at the Edge." Zoglin has it all over Knoedelseder when it comes to rich, contextual understanding of just how important those comics were (it is their tone that prevails in blogworld and political satire on cable TV), but it is Knoedelseder who has the genuinely inside information about a virtually hidden event that turns out to be a pivotal one in American culture, the 1979 "strike" (more like a boycott) of L.A.'s Comedy Store by just about every stand-up comic of the time who worked there.
That's when Jay Leno -- not Letterman -- changed a little bit of showbiz history by performing, for real, a kind of NBA player's flop in the parking lot of the Comedy Store.
The strike was a genuinely bitter one. Comics picketed to get paid. Comedy Store owner Mitzi Shore -- who had cradled and showcased half the comics America still loves (not to mention those who influenced them) -- wouldn't give in.
All the comics we still know were on the striker's side, including those, like Letterman, who owed Shore the most. None of those on her side -- Argus Hamilton, Biff Maynard, her then-boyfriend Steve Landesberg -- are of much consequence now.
It was Maynard whose car sped into the Comedy Store lot and came close enough to Leno to prompt him to do a convincingly exaggerated flop and howl in "pain" supposedly on the way to the hospital.
It was a ploy to inspire the troops by one of the strike leaders and it worked.
In other words, it's -- sort of -- Leno's bit. He did it more memorably than anyone else. Which explains why it hasn't been so prominent on Letterman lately.
You have absolutely no idea how close Leno and Letterman once were until you read this book. They were good friends, mutual admirers, students of comedy and comic activists together long before they were rivals.
But then the list is very long of things you probably didn't know until reading this book.
That, for instance, Elayne Boosler was, in her way, almost as crucial a figure for female comics as Joan Rivers and Phyllis Diller (she was also among the most militant anti-Mitzi comics). That Boosler once had a thing with Andy Kaufman.
And that there are good reasons why Letterman maintains so much loyalty to comedy pals like Tom Dreesen and Jimmie Walker despite career arcs that wouldn't seem to warrant it.
They were major league good guys, benefactors and heroes in the comics' cause.
Another almost forgotten good guy is David Brenner, now finally admitting to 72, who turns out to have been a kind of freelance big brother to all manner of struggling comics in need (including, among others, Richard Lewis).
And as a kind of parallel to Leno's morale-boosting fakery, there is the solidifying and very real horror of the fate of hapless Steve Lubetkin, a comic who kept missing the main chance and finally went off the deep end after the strike ended.
The comics had supposedly been triumphant and won pay for themselves. Even so, Lubetkin, and others of lesser consequence, found themselves frozen out of spots at the Comedy Store.
His name is virtually guaranteed to be unknown to you. It is, on the other hand, just as guaranteed to be known to almost every working stand-up comic in America.
At dinnertime on June 1, 1979, Lubetkin went to the roof of the Continental Hyatt House next door to the Comedy Store. He jumped and dropped 105 feet to his death on the parking lot below.
Whether he was hoping to actually hit the Comedy Store, we'll never know.
There was a bloody suicide note in his jeans, leaving messages to his good friend Richard Lewis, his fellow comic picketers ("I guess nice guys do finish last"), to all comedians ("unite, it's in your best interest") and to the world ("fairness, fairness, fairness, please, before it's too late").
A little-known story has now been told very well in perfect context. And when you finish the book you may feel as if you finally understand every comedian you see on TV for the first time.