Fiona stands on a smooth, flat playground holding a large helium balloon on the end of a string. She lets go of the string. What happens next to the balloon and why?
Make your response very short.
The helium balloon floats upwards after Fiona lets go of the string. This happens because helium is lighter than air; it is less dense. The balloon filled with helium is buoyant in air, which means it rises until the external pressure of the surrounding air equals the internal pressure of the helium in the balloon. The string, no longer held down by Fiona, follows the balloon upwards due to being attached to it.