Why do we get déjà vu and what causes the sensation?

We get déjà vu when our brain thinks it’s seeing something it already knows, even though it's not really familiar.

Imagine you're playing with your favorite toy, a red ball. You see the ball roll across the floor and you go to catch it, but for just a moment, it feels like you've seen this exact same thing before, maybe in a dream or at the park yesterday. That’s déjà vu!

Sometimes, your brain gets mixed up. It sees something new, but part of your memory thinks it’s old. Like when you're eating a sandwich and suddenly feel like you've had this exact same sandwich before, even though you haven’t! Your brain is trying to match what you’re seeing with what it remembers.

It's kind of like having two friends who both know the same song, but they start singing different parts at the same time. One part of your brain says "this looks familiar," and another part says "no, it's new." For a moment, you feel confused, that’s déjà vu!