Our culture is an edifice built of externalized memories.

The best memorizers in the world - who almost all hail from Europe - can memorize a pack of cards in less than a minute. A few have begun to approach the 30-second mark, considered the 'four-minute mile of memory.'

With our blogs and tweets, digital cameras, and unlimited-gigabyte e-mail archives, participation in the online culture now means creating a trail of always present, ever searchable, unforgetting external memories that only grows as one ages.

As bad as we are at remembering names and phone numbers and word-for-word instructions from our colleagues, we have really exceptional visual and spatial memories.

I met with amnesiacs and savants, educators and scientists, to try to understand what memory is, why it works, why it sometimes doesn't, and what its potential might be.

Evolution has programmed our brains to find two things particularly interesting, and therefore memorable: jokes and sex - and especially, it seems, jokes about sex.

The more we remember, the better we are at processing the world. And the better we are at processing the world, the more we can remember about it.