Do walls have ears?

1. Put a mark on the wall of your classroom from inside and outside, just opposite to each other.
2. Ask your friend to go outside the classroom and hit the wall at the mark with a small pebble.

3. You, try to listen to it by putting your ear against the same spot on the wall from inside the of the classroom.

4. Hit the wall gently so that your friend (behind the wall) cannot hear the sound directly.

5. Now change the role and try again.
 

Were you able to listen through the wall?