Joke:What is a joke?
Question : What lies at the bottom of the ocean and twitches?
Answer : A nervous wreck.
To twitch means to move a part of your body very quickly every few seconds. Some people have a twitch in their face which makes their mouth, for example move quickly every few seconds. People often twitch when they're nervous.
Wreck has two meanings : a ship that sinks and ends up at the bottom of the sea is called a wreck. We also use the term nervous wreck to describe someone who cannot function normally because they have too much stress or are too worried.