From the above picture you can see that the train has stopped. It has stopped because of the signal which says 'stop'. Similarly, a fullstop '.' also says, "Stop, the sentence is over. Pause and then read the next sentence."
The words fat and thin do not have the same meaning.They have opposite meanings.
Words which have opposite meanings are called Opposites.
The sign X is used to show oppposites. eg fat X thin.