A radio works by receiving an electromagnetic signal from the radio tower. This signal is n electromagnetic wave made from oscillating electric and magnetic fields. (It a lightwave we cannot see.) If ono or both of these fields is stripped from a wave, the wave will cease to exist.
To show that the electric field is zero insideFaraday cage, place a small FM radio inside of a "cage" made from aluminum. When this is done the readio signal is blocked to the point where is cannot be picked up anymore.
However, this is not perfect. A strong enough signal or a different frequency can power past the small openings. Students will probably ask to put their cellphone in the cage. Ut oh. The cellphone works! It can receive a signal. You need a better, more solid, Faraday cage.
Instead, either use a cookie tin,
or wrap it in aluminumfoil.
Now the phone will not receive any signal.