When I was at medical school we were taught that we were born with all the brain cells we would ever have, and that after middle age it was downhill from then on.
But we now know this isn't true.
Brain-scan technology means that we can now peer inside the living brain and take a look at how it functions.
Research is shedding light on something that scientists call 'neuroplasticity' - the idea that our brains keep on changing as we age.
We go on growing new brain cells and making new brain connections throughout our lives.
What's more, we now know far more about the extent to which these changes are influenced by the world around us, and even by the choices we make in our day-to-day lives.
BBC Horizon and Dr Adam Hampshire at Imperial College, London, want to explore how our changing behaviour and lifestyle could be affecting our brain function.
And you could help by taking the Great British Intelligence Test.
We'll be presenting the overall results of our test in a special episode of Horizon, which will be broadcast on BBC2 in the coming months.
Why not give it a go, and get immediate feedback on your own mental strengths and tips for improvements?
Among other things, I'm hoping The Great British Intelligence Test will show us what we can do, not only to maximise, but also to protect our brain power.
Like many middle-aged people, I worry about the fact that I find it harder to remember names and details as I get older, and I'm disconcertingly aware that I rely on my smartphone to remind me of phone numbers and diary appointments. I want to understand why my memory is getting worse with age, and what I can do to improve it.
I also want to find out how our ever-changing world could be affecting our cognitive function.
Over the last few decades the internet, smartphones and social media have utterly transformed the way we absorb information and interact with one another.
In The Great British Intelligence Test, we'll be able to take a look at how this explosion of technology could be affecting our brains, and what it might mean for our intelligence - both now and in the future.