A retroflex click is a sound that your mouth can make. You make it by using your tongue and the roof of your mouth. Your tongue curls back a little bit and touches the roof of your mouth. Then you pull your tongue down quickly, making a clicking sound. It's kind of like the sound you make when you snap your fingers, but it's made inside your mouth. People in some languages use this sound as part of their language to communicate with each other. It might be hard to make this sound at first, but with practice, you can learn to do it too!