Let them hold it, smell it, realise it's wax. Then ask what they thought it was. They'll say: "A candle."
They can't remember being fooled — five seconds ago.
One truth at a time. Little kids can't hold two versions of reality at once. Once they know it's a candle, the old belief — "apple" — is gone. To them, they've always known.
Other minds are invisible. Show a crayon box filled with candles. Ask: what will your friend outside think is inside? A five-year-old says "crayons" — they get it. A three-year-old says "candles." They can't imagine someone else not knowing what they now know.
Theory of mind is the ability to understand that other people have different thoughts, beliefs and knowledge. It switches on around age four. You weren't born with it.