Are we living in a simulation?

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Is the whole universe just a simulation? —Moumita B., age 13, Dhaka, Bangladesh

How do you know anything is real? Some things you can see directly, like your fingers. Other things, like your chin, you need a mirror or a camera to see. Other things can’t be seen, but you believe in them because a parent or a teacher told you, or you read it in a book.