Betelgeuse and supernova?

Imagine that next winter you are stargazing with your boyfriend/girlfriend. In order to impress him or her, you point out the bright star Betelgeuse in Orion. You tell him/her: "When Betelgeuse dies, it will explode in a supernova. In fact, it might have already done that." Explain how that's possible, when Betelgeuse still looks the same to us.