Scientists have theories, of course, and one of them is that when you experience something for the very first time, more details, more information gets stored in your memory. Think about your first kiss.

Neuroscientist David Eagleman of Baylor College of Medicine says that since the touch of the lips, the excitement, the taste, the smell — everything about this moment is novel — you aren't embroidering a bank of previous experiences, you are starting fresh.

Have you noticed, he says, that when you recall your first kisses, early birthdays, your earliest summer vacations, they seem to be in slow motion? "I know when I look back on a childhood summer, it seems to have lasted forever," he says.

That's because when it's the "first", there are so many things to remember. The list of encoded memories is so dense, reading them back gives you a feeling that they must have taken forever. But that's an illusion. "It's a construction of the brain," says Eagleman. "The more memory you have of something, you think, 'Wow, that really took a long time!'

"Of course, you can see this in everyday life," says Eagleman, "when you drive to your new workplace for the first time and it seems to take a really long time to get there. But when you drive back and forth to your work every day after that, it takes no time at all, because you're not really writing it down anymore. There's nothing novel about it."

That may be because the brain records new experiences — especially novel and exciting experiences — differently. This is even measurable. Eagleman's lab has found that brains use more energy to represent a memory when the memory is novel.

So, first memories are dense. The routines of later life are sketchy. The past wasn't really slower than the present. It just feels that way.