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Michael: You might be in hundreds of photos in your life, at birthday parties, graduations, or just snapping a selfie with friends. But the very first photo of you was probably taken while you were still inside your mom – thanks to the power of ultrasound.

Doctors can use an ultrasound transducer to make ultrasound waves that travel through your body, and reflect off different squishy bits, like organs, or a fetus. Then, the transducer uses the reflected waves to create an image. That cold, sticky gel gets smeared on your body to help these waves make it to those inner tissues. But how exactly does it work?

Well, ultrasound waves are just another type of sound wave. And sound waves travel by vibrating particles back and forth, like the ones that make up air. The higher the frequency of a sound, the faster the vibrations. At really high frequencies – around 20 kiloHertz – the vibrations can’t be picked up by human ears, so we call them ultrasound waves. Because ultrasound is just... sound, it’s really safe for medical imaging.

When sound waves travel from one medium to another, like from air to your skin, or skin to your liver, they can be partially reflected. And these high-frequency, short-wavelength ultrasound waves are especially good at reflecting without spreading out too much. That’s why they’re good for making detailed images, and for echolocation if you’re a bat or dolphin.

But how do doctors make sure the waves travel into your body to reflect off organs, instead of just bouncing off your skin? Here’s where ultrasound gel comes into play. Something’s acoustic impedance – basically, how much it resists vibration due to sound – affects how much a sound wave will reflect when it travels between two materials. If there’s a huge change in acoustic impedance, like from air to human skin, sound waves will mostly bounce off.

But ultrasound gel is a mix of water and polymers like glycerin and propylene glycol, and has an acoustic impedance that’s really similar to human skin. By using a transducer to make ultrasound waves in that gel, the waves have a much better chance of transferring into your body, with less reflection right off the bat. As the waves travel deeper, some will reflect every time there’s a change in tissue type, like from the lining of a womb to tiny baby fingers.

The transducer keeps track of things like how long the reflected waves were traveling, and the frequency and amplitude of the wave. Then, a computer uses all this information to generate images of what’s inside you. All thanks to sounds that you can’t even hear.

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