It’s all thanks to your vocal cords. These are muscles that stretch like rubber bands across your voice box, or larynx (LAIR-inks), which is located at the back of your throat. Whenever you say hello to a friend or answer a question in class, you’re using your vocal cords to speak. The size of your vocal cords is what determines your voice’s pitch, which is its level of highness or lowness.
When you speak, air pushes from your lungs through your vocal cords and out of your mouth. The air makes the vocal cords vibrate, or move back and forth really fast. Picture the strings on a guitar. The vocal cords' vibration makes sound, just like a strummed guitar does. And like guitar strings, smaller cords create a higher sound. Girls' vocal cords are usually shorter and thinner than most boys’, which is why their voices sound higher. This difference in pitch is even more noticeable in grown-up men and women.
When you get a little older, you'll experience something called puberty (PYOO-bur-tee). It's your first step towards being an adult. Boys' bodies start producing a lot of testosterone (tes-TOSS-tuh-rone). This is a hormone. It acts like a chemical messenger to different parts of the body. Testosterone tells the body to change in all sorts of ways, like making bigger muscles and growing hair in new places. It also makes a boy's vocal cords grow thicker and longer. That makes his voice sound deeper. Girls' bodies produce testosterone too, but not as much as boys' do. Their bodies also produce another hormone called estrogen (ESS-tro-jen). These hormones make girls' vocal cords get bigger during puberty, but they don't grow as big as boys'. So girls' voices sound higher.
Have you experimented with making your voice sound different? When you speak in a really low voice you’re contracting your vocal cords to make them thicker. When you talk in a really high voice you're stretching them to make them thin. Do you want to see what this looks like? Stretch a thick rubber band and pluck it. What does it sound like? Now stretch a thin rubber band. I bet it sounds different!
For further reading on your voice and how it changes, visit Kids Health.