What is something you can fill a balloon with?

Answer 1

You can fill balloons with gasses or liquids. Lighter-than-air gasses such as helium are the best if you want the balloon to float upwards. Ordinary air is no use as it is denser than the surrounding air, making the balloon too heavy to float upwards (it will float downwards). However, hot air is less dense than cold air and therefore lighter than air.

All liquids are far too heavy to float, but water balloons are generally intended to be thrown, typically at people. Water balloons are typically made of thinner rubber than air balloons thus when the balloon strikes the person, the balloon's envelope bursts, soaking the person. Water balloons also have more practical uses, such as in Foley catheters. Here, the catheter is inserted into the bladder through the urinal tract, where a balloon is inflated with sterile water to prevent the catheter from falling back out of the bladder. A valve on the end of the catheter allows the balloon to be inflated or deflated for removal.
Two things are water or helium.