51动漫

When I can鈥檛 remember something, how do I know that I ever knew it in the first place?

John Ripley, Leeds, West Yorkshire, UK

We pay 拢25 for every answer published in 51动漫. To answer this question 鈥� or ask a new one 鈥� visit newscientist.com/lastword. Terms and conditions apply.

This article appeared in print under the headline 鈥淭otal recall鈥�