Its some kind of charm picking up stones on the beach? Im curious about it because recently i went to a Beach in 日照 an i saw a lot of people picking up stones

Chinese sayings use 4 words like 莫名其妙,any more examples? And the meaning please

When a chinese says that my chinese is good and i answer 哪里哪里,they tend to laugh.Whats so funny? Thanks all people to answer.Now i understand i bit more about chinese customs