But what if you kept going? Where would you end up?" It's the question we ask of our Dads around the time of our 10th birthdays.
We're looking up at the night sky in awe and pose what seems like a perfectly simple question, but is actually a complex puzzle of where human life sits within the cosmos.
It is around this time that you start to realise your Dad doesn't know as much as you'd hoped. This process ends at 18, when you realise he knows very little at all.
At 10, he will blither on about planets and far away stars, but when you ask, "And after that?", it's all a fluster and a bluster.
Words like Dark Star and Vortex might make it through the mumbling if he's one of the clever ones, but it doesn't really help.
I was thinking about this while watching one of those brilliant BBC science programmes the other day. It was the sort you would sign a petition to save from the axe, even though you hardly ever watch them.
In the last five years, I've seen about one-and-a-half, but every time I emerge with a feeling of self-satisfaction as I parade my new half-knowledge to the imbeciles who stuck with Come Dine With Me.
This one was about two miraculous little space ships called Voyager 1 and 2.
In 1977, Nasa sent the two, the size of an average family saloon, on a never-ending mission to distant worlds. They would visit the furthermost planets in the solar system, Jupiter, Saturn, Neptune and Uranus, sending back vital data and being bombarded by space dust and radiation for their troubles.
On board, two discs of messages for any alien life forms they came across, containing pictures of people with middle partings and flares, singing bad songs, were stored.
This has probably saved the planet from hostile alien invasion on the grounds that any species wearing wide-striped corduroy was hardly worth colonising. Down on earth, boffins (but still wearing their hair parted in the middle and sideburns, even the women) lovingly nurtured these two little craft on their journey.
And, reader, the Voyagers program is the space mission that keeps on giving. Vital information about the gases that form our solar system have given scientists crucial insights into the vast place in which we live, as well as an Everest of stuff on other space issues too complicated to go into here.
And here's the rub. They are still going. Sending out little beep, beep, beep messages to those same scientists, now bald (even the women) whose love for the little craft appears to match that for their children and grandchildren who have come into the world since they waved goodbye to 1 and 2, 35 years ago.
You have to wonder if the Voyagers ever worry that their little tweets of data (which now take 24 hours to reach us and are getting fainter) are still being received. For all they know, Nasa could have packed up and gone home.
Now the exciting part. With nothing more powerful to drive them than that which runs the laptop on which I'm writing this piece, Voyager 1 is reaching the outer limits of our solar system, some 11 billion miles from the Sun.
It will soon cross, and I quote from the Nasa website, 'the heliosheath into interstellar space'.
In Star Trek-speak, it's the final frontier, except Voyager will keep going.
Some day, it will lose power and be lost to us, but before it does it may give us answers to questions we don't even know how to ask.
In the meantime, that 10-year-old's unanswered question remains at the core of our existence. It's what keeps us in wonder.