There’s a scientific explanation to this paradox: when the carton is almost full and we pour from the top of it, the juice never covers all of the opening. There’s air on top of the juice, which balances the pressure of the surrounding atmosphere. The juice can therefore flow out smoothly using gravity. But when we’re trying to pour juice with the bottom of the carton angled up, the liquid covers the opening completely. The air pressure conflict between the atmosphere and the air trapped inside the carton, will push the juice back into the carton.