How do all our memories fit in?

I've been doing a bit of yoga recently, trying to clear my head, put some good thoughts there and remove some of the worries. This has got me thinking about our thoughts and our memories. What makes memories linger in our brain? I'm not talking about bad memories here, not today. I'm just talking about the good ones, the kind that can be replayed over and over in your head, the kind that can take you to a better place when you're feeling fed up.

What amazes me is that there seems to be no limit to how many memories any one brain can hold. The popular image of a memory is that of a filing cabinet, where each memory is stored away in some kind of sensible order. That's how I like to think about mine anyway and it will come as no surprise to my blog friends that a lot of my memories appear to be filed away under the heading of 'pop songs'. It's not only music that can bring memories to life, though. Smells, tastes, even colours can be evocative of times past.

I remember from my GCE Biology (yes, they were called GCEs in those days before GCSEs had been invented!) ...I remember that memories are stored in the hippocampus but that doesn't help me to understand how it works because the hippocampus is a very small part of the brain. How do all the memories fit in? I also remember learning that the average brain weighs about 3 lbs and 80% of that is made up of water. Mum helped me to revise Biology and we did it so well that a lot of this stuff is still stored in my memory. Why is it not overflowing?

What's more, our memories can be 'jogged'. Go to a reunion or meet up with an old friend and a whole file full of memories can be reopened. You start thinking about things you did all those years ago, things that you'd almost forgotten about, but now the memory has been 'jogged', they're as vivid as if they happened yesterday.

Of course, the bad news is that our memories can start to lose their 'search facility' as we get older. That old chestnut of walking into a room and forgetting why you're there happens to most of us and it's apparently because by the time we reach 50 the connections between neurons in our brains are starting to show their age.

I'm not sure how scientifically effective it is but I'm doing my best to keep my brain active in the hope that it will maintain its full search facility functions for as long as possible. I play Bridge at least once a week and I play word games and do crosswords every day, although the Guardian cryptic crossword is much tougher these days than it used to be - humour me! It is tougher, isn't it?!

What do you do to keep your brain active?