Normally, when singing, you can adjust the pitch of the tone that you create based on the positioning of your tongue in your mouth, which is a form of air chambering that increases or decreases the tone.
You also might have noticed that you can only sing one tone at a time; but is this always the case?
Anna-Maria Hefele is a German singer who is able to actually sing two separate tones at one time. The tone that you hear for the most part is the base part of her voice that you typically create when you sing, but you can also hear a higher-pitch tone, which she is actually manipulating with her tongue in a unique way. This is known as a polyphonic overtone singing.
Doing this is actually pretty hard to do. It takes years of practice coupled with a good understanding of the muscles in your tongue as you produce sound with your vocal chords.
Maybe with a little practice, you could sing like Hefele.