Alex and Andrew live in one of the coolest cities ? make that the hottest? They live in Albuquerque, New Mexico, home of one of the biggest hot air balloon festivals anywhere. They hope to get up in one of those giant balloons someday, but in the meantime, they're doing some investigating on a smaller scale. They visited their favorite science hangout, Explora, to find out: How big does a hot air balloon need to be to lift things off the ground?