Saying thank you
You need to give thanks to the people who gives you gifts... It takes effort and time to decide or to make the gift
Say thank you
People will appreciate even the smallest things like saying thank you, because it will show that you appreciate the effort of that someone of giving you smth.
the scientific reason why empty cans make the loudest noise because empty cans has more space inside, allowing sound to echo loudly. applying that to people, the meaning of the idiom is that people with empty brains (stupid or have no knowledge) makes the loudest noise (by chatting with their seatmate or just being overall noisy).