If the little grey cells are not exercised, they grow the rust.

"A sound of laughter was heard-they turned sharply. Vera Claythorne was standing in the yard. She cried out in a high shrill voice, shaken with wild bursts of laughter:
"Do they keep bees on this island? Tell me that. Where do we go for honey? Ha! ha!"
They stared at her uncomprehendingly. It was as though the sane well-balanced girl had gone mad right before their eyes. She went on in that high unnatural voice:
"Don't stare like that! As though you thought I was mad. It's sane enough what I'm asking. Bees, hives, bees! Oh, don't you understand? Haven't you read that idiotic rhyme? It's up in all of your bedrooms-put it there for you to study! We might have come here straightaway if we'd had sense. Seven little soldiers chopping up sticks. And the next verse, I know the whole thing by heart, I tell you! Six little soldier boys playing with a hive. And that's why I'm asking-do they keep bees on this island- isn't it damned funny...?

What are murderers like? Some of them, have been thoroughly nice chaps.