Think about things that seem to float in outer space. Scuba divers who wear wetsuits can float to the surface very easily. To stay under water, they often have to wear extra weights. If the balloon started to sink, the pilot threw sand overboard. Today, pilots of sport balloons regulate floating and sinking by adding or releasing hot air.
Astronauts practice underwater to learn how to work in the weightlessness of space.
You know that if you figured out how many helium balloons you would need in Balloon Ride. Remember, one small metal paper clip weighs 0.25 g. 1996, National Air and Space Museum, Smithsonian Institution.