Touch sensors use capacitive sensing to work. Capacitors are a basic part in electronics because they can store up electrical charge and then release them. They do this by  storing a charge between two conductive plates. Capacitive sensing involves a lot of science to understand how dielectric materials work with electrostatic fields, electromagnetism, and insulators. In our capacitive sensor, the conductive material (the copper tape) acts as one plate and everything else (the open air) acts as the second plate. We can measure the capacitance of this system: when we bring our hand close to the sensor we interact with the field it is generating and become the second plate changing the capacitance of the system. We can measure this change and use it to trigger an action on our microcontroller. In your sensor, how strong do you think the magnetic field is? Strong enough to go through paper or glass?