word puzzle

hello.
I am trying to write a word puzzle(letters on a big square and you need to find specific word in there) and I want the game to draw dinamiclly the ovel around a word the player choose. When the player click with the mouse on a letter the oval will start and will grow as the mouse movement continue until a mouse release. If the player is right, the oval stay on the board, else it will be erased.
thanks for someone who can help.