Vestigial response

Okay kiddo, you know how sometimes when someone tickles you, your body responds by laughing even though you didn't decide to laugh on purpose? That's because your body has a reflex or response that is left over from a long time ago when it was really important for your survival.

But now that reflex isn't really that useful anymore, it's just something your body does without really thinking about it. That's what we mean when we say it's "vestigial" - kind of like a leftover or a relic from the past.

So a "vestigial response" is when our body does something automatically that used to be really important but now it's not so useful anymore. It's like a little reminder of our evolutionary history.