Science Buddies: "Ask an Expert"

I found it very interesting to read this STEM lesson. I have a question, what does the structure of the Harmonica have to do with students' products? Can you explain more clearly about the physics knowledge used here? Thank you so much
-------------------------------------
Leave this to help the Experts:
The activity can be viewed at: Make Your Own Harmonica!

Hi - This activity has two explanatory sections at the bottom. The Digging Deeper section helps explain how the design of even this simple harmonica creates vibrations at different frequencies.

Amy
Science Buddies