Illusionist Derren Brown, who has previously weighed in on the psychology of gullibility, answers 5-year-old Evie’s question about how we can be sure that life isn’t just a dream, touching on the limits of our perception of “reality”:

Often we have dreams and they feel so real that we might wonder whether we’re dreaming right now too. It feels like you’re wide awake now, but doesn’t it feel like you’re wide awake in dreams too? How on Earth can you tell the difference? Maybe you’ll wake up in a moment and realize you weren’t reading this book — because it never existed!

Well, at least you know you’re probably real. Because even if you were having a dream right now, there would have to be a you somewhere who was having that dream about yourself. But before your head starts spinning too fast, here’s the important thought. We only ever really know about the stuff we see and hear and feel, and that’s only a tiny part of what’s around us. (For example, you can’t see what’s happening in the next room, or in someone else’s head.) We can only guess at what’s real from the little bit we know about — and often we get it very wrong. … So even though you’re probably not dreaming, it’s worth remembering that you’re only aware of a small part of what’s real, too.

(Meanwhile, it’s been argued elsewhere that the probability that you are dreaming right this moment is 1 in 10.)

Wait, there's a 10% chance that I'm dreaming right now?!

I'm not dreaming now. 

I think.