Holding breath.
My sister and I have always been stubborn and fairly competitive, fighting over who got a bigger portions of things (our parents didn't know the one cuts, one chooses thing), who could run faster, who made better Lego toys, that sort of thing. Even though she's nearly three years younger than me, she was only very slightly shorter than me throughout most of our childhood and overtook me when I was around 15 (she 12), with the result that when I first sneaked into a pub and drank beer underage, she did the same a couple of weeks later! But that's Essex for you...

Anyway, on car journeys we used to hold our breath going through tunnels.

It was something to do, I suppose. We'd see a tunnel coming and try and hold our breaths all the way through it. Most tunnels were short enough that we knew we'd succeed, but we'd still sit in the back of the car staring at each other, cheeks bulging, eyes going slightly bloodshot, each checking that the other's breath was still being held. My parents got used to the "poooossshhhh" sound of two rapidly exhaling children that coincided with re-emerging into daylight.

The trouble is, we still do it. But now we're usually the ones driving.

She's got used to it now, but my wife has been made shouty-angry on several occasions when I've taken a few deep breaths when approaching a tunnel and ceased conversation with her to puff my cheeks out, hold my breath and try and make it through the tunnel. The Blackwall and Dartford Tunnels are a particular trial if there's a lot of traffic, and I've occasionally hyperventilated enough before going into the tunnel that my peripheral vision has gone a bit wavy and sparkly and I've had to _really_ concentrate on driving properly.

As a grown-up, I know it's a silly idea. My sister lives around 200 miles away, we're very rarely even in the same car any more, but we both still habitually do it. We've both got to know the tunnels in our area by the rough speed that you need to keep up in order to hold your breath all the way through and we've both seriously wound up our respective spouses by not only stopping conversation but also driving a little faster than we should and getting tense and starey as we progress, ending each tunnel with a relieved "pooosshhh" sound and resumption of conversation as if nothing had happened.

And we'll still compare notes every now and then when we meet up, guiltily taking pleasure in how stupid and potentially dangerous the childhood habit now is.

My best was the Dartford Tunnel at just under 40mph.
( sanityclause is waiting until, Fri 21 Nov 2008, 12:31, 3 replies)

haha!
yes, I do the holding-breath-in-tunnels myself.
Mostly when on the bus, as I don't trust myself not to pass out if I do it whilst driving...
(The Empress it seems you CAN polish a turd!, Fri 21 Nov 2008, 12:53, closed)

I haven't passed out yet
but things have gone a bit sparkly or I've got to the twitchy, blowing-out-as-slowly-as-possible stage. Mostly when driving on my own, predictably. I still have that internal "must-outlast-my-sister" thing going on...
( sanityclause is waiting until, Fri 21 Nov 2008, 13:15, closed)

rather than holding breath
my brother and I tend to go "AAAAAAAAAHHH" all the way through the tunnel

equally good fun
( Vipros. clever got me this far, then tricky got me in, Fri 21 Nov 2008, 14:00, closed)