On this weekly show , we tell about commonly used American expressions . By knowing these expressions , you can improve your ability to speak and understand American English . Today , we go deep into the high seas to meet one of the most feared hunters in the ocean – the shark ! Sharks are large , usually gray saltwater fish . Unlike other creatures , they have a skeleton made of cartilage , not bones . Sharks also have sharp teeth . They are known for being dangerous and eating whatever they want . That's in the water . On land , Americans use the word “shark ” in a number of expressions . It's no surprise that they all suggest some form of danger . The word shark can be used to describe someone who is tricky and uses other people . An example of this is the term loan shark . Loan sharks are people , not fish . They loan money at very high interest rates to others who need the money badly . A “shark ” can also be someone who is very good at something . However , they often use their skill to trick others . Sometimes we use this version of “shark ” as a verb . For example , let's say you are skilled at playing the game of pool , also known as pocket billiards . In fact , you never lose a game . One night , you go to a pool hall and act like you are not a good player . You invite another person to play a game . You play against them and lose . Then , you ask them to give you another chance . Not only that , you offer them money if they beat you a second time . The opponent happily accepts and , to be fair to you , makes the same offer . But you defeat them and win all the money . In a situation like this , you are considered a pool shark and have “sharked ” your opponent . But you had better leave . As at the beach , sharks are also not popular in a pool hall . It goes without saying that sharks , whether in the water or on land , can be dangerous . If you are swimming with sharks in the ocean , you might be in serious danger . And if you are swimming with sharks on land , you are spending time with the wrong crowd . Let's go back to the pool hall and our story about the pool shark . Imagine that the people whose money you won are not the nicest . They are criminals who are now angry at you for winning their money . You could say , based on this example , you are swimming with sharks . When sharks smell blood in the water , they come and circle the wounded animal . The same can be said for people . Let's say you are at work and experience bad luck on a project or two . You are in danger of losing your job . Your co-workers may smell blood in the water . You could say the sharks have started circling . In this example , your mistakes are the blood and the sharks are your co-workers . On the other hand , someone who is shark bait is not dangerous -- just the opposite . Shark bait is one way of describing a person who swims in the ocean alone . But for everyone else , shark bait describes someone who is unsuspecting and easy to trick . Two great words that mean “shark bait ” are naïve and gullible . Now , to jump the shark is one of those idioms that doesn't make much sense unless you know about its roots . The expression “jump the shark ” comes from the American television show “Happy Days .” This TV series was first broadcast in 1974 and continued until 1984 . In 1977 , Fonzie -- the show's most popular character -- jumped over a shark while waterskiing . It was a silly story . And years later , TV critics said this jumping the shark program was the beginning of the decline of “Happy Days .” So , when Americans say something has “jumped the shark ,” they mean it used to be popular or of high-quality , but no longer is . We also use this expression to describe something that has become silly or has no connection to reality . This expression usually refers to a TV show or the career of an artist or writer . Not everyone believes that Pablo Picasso's art jumped the shark later in his career . But some critics think that it did . … and I'm Jonathan Evans .