Have you ever had a dream that felt so real you were surprised to wake up? Philosophers ask a puzzling question about this: how do you know you are not dreaming right now?

In a dream, strange things can feel totally normal. You might fly, talk to a dinosaur, or be in two places at once, and while you are asleep it all seems perfectly fine.

When you wake up, real life has clues that dreams usually do not. In real life, things follow steady rules: you cannot float to the ceiling, and the clock keeps normal time.

One clue thinkers use is that real life stays put. If you look at a page of words, look away, and look back, the words are still the same. In dreams, words often change or turn to nonsense.

Another clue is that real life connects together. Yesterday leads to today, and you remember how you got where you are. Dreams often jump around with no clear path from one part to the next.

You do not have to solve this puzzle to enjoy it. Just wondering how you know you are not dreaming right now is exactly the kind of deep question philosophers love to explore.