Question: why do sometimes we feel that we have lived that exact moment before?

Susan Cartwright answered on 16 Jun 2015:

This experience is known as deja vu (French for “already seen”). It is actually not at all well understood. People with certain types of epilepsy reliably experience deja vu during or just before the seizure, so the likelihood is that it is due to some minor malfunction in the brain. One possibility is that the new episode has enough similarities to something you’ve already experienced that it mistakenly activates the memory sequence from the previous experience, causing you to feel that you’ve done it before even though you haven’t. Another is that the process of storing the new memory gets slightly mistimed, so that something that is actually in the present feels as though it’s in the past. As I say, nobody really knows, and this is the subject of active research.