It would be very cool to ride around using jet packs, but unfortunately some of Earth's physical properties limit our use of them. Unlike birds, humans are not aerodynamic meaning that it very hard to keep us afloat. To do this, we must solely rely on thrust and to generate this thrust, a large amount of fuel is needed. Unfortunately, the more fuel needed, the heavier the jet pack becomes. This requires more fuel to lift the weight and the problem just continues. Because of this dilemma, jet packs are not very practical and many last less than a minute in the air. In order to make them more efficient, scientists will have to find a way to make jet packs more fuel efficient.