“I read once that, on average, we replace the majority of our cells every seven years. Even more amazing: we change the upper layers of our skin every two weeks. If all the cells in our body did this, we’d be immortal. But some of our cells, like the ones in our brains, don’t renew. They age, and age us. In two weeks my skin will have no memory of Olly’s hand on mine, but my brain will remember. We can have immortality or the memory of touch. But we can’t have both.”