Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: A balloon is filled with Helium, and then is opened and pressed up to someone's mouth to make a voice high, why does it do this?
gas fills the new area
Helium makes voices high because it is a lighter-than-air gas.